diff --git a/app/models/spree/adjustment.rb b/app/models/spree/adjustment.rb
new file mode 100644
index 0000000000..8d398860fc
--- /dev/null
+++ b/app/models/spree/adjustment.rb
@@ -0,0 +1,177 @@
+# frozen_string_literal: true
+
+require 'spree/localized_number'
+require 'concerns/adjustment_scopes'
+
+# Adjustments represent a change to the +item_total+ of an Order. Each adjustment
+# has an +amount+ that can be either positive or negative.
+#
+# Adjustments can be open/closed/finalized
+#
+# Once an adjustment is finalized, it cannot be changed, but an adjustment can
+# toggle between open/closed as needed
+#
+# Boolean attributes:
+#
+# +mandatory+
+#
+# If this flag is set to true then it means the the charge is required and will not
+# be removed from the order, even if the amount is zero. In other words a record
+# will be created even if the amount is zero. This is useful for representing things
+# such as shipping and tax charges where you may want to make it explicitly clear
+# that no charge was made for such things.
+#
+# +eligible?+
+#
+# This boolean attributes stores whether this adjustment is currently eligible
+# for its order. Only eligible adjustments count towards the order's adjustment
+# total. This allows an adjustment to be preserved if it becomes ineligible so
+# it might be reinstated.
+module Spree
+  class Adjustment < ActiveRecord::Base
+    extend Spree::LocalizedNumber
+
+    # Deletion of metadata is handled in the database.
+    # So we don't need the option `dependent: :destroy` as long as
+    # AdjustmentMetadata has no destroy logic itself.
+    has_one :metadata, class_name: 'AdjustmentMetadata'
+
+    belongs_to :adjustable, polymorphic: true
+    belongs_to :source, polymorphic: true
+    belongs_to :originator, polymorphic: true
+    belongs_to :tax_rate, -> { where spree_adjustments: { originator_type: 'Spree::TaxRate' } },
+               foreign_key: 'originator_id'
+
+    validates :label, presence: true
+    validates :amount, numericality: true
+
+    after_save :update_adjustable
+    after_destroy :update_adjustable
+
+    state_machine :state, initial: :open do
+      event :close do
+        transition from: :open, to: :closed
+      end
+
+      event :open do
+        transition from: :closed, to: :open
+      end
+
+      event :finalize do
+        transition from: [:open, :closed], to: :finalized
+      end
+    end
+
+    scope :tax, -> { where(originator_type: 'Spree::TaxRate', adjustable_type: 'Spree::Order') }
+    scope :price, -> { where(adjustable_type: 'Spree::LineItem') }
+    scope :optional, -> { where(mandatory: false) }
+    scope :charge, -> { where('amount >= 0') }
+    scope :credit, -> { where('amount < 0') }
+    scope :return_authorization, -> { where(source_type: "Spree::ReturnAuthorization") }
+
+    scope :enterprise_fee, -> { where(originator_type: 'EnterpriseFee') }
+    scope :admin,          -> { where(source_type: nil, originator_type: nil) }
+    scope :included_tax,   -> {
+      where(originator_type: 'Spree::TaxRate', adjustable_type: 'Spree::LineItem')
+    }
+
+    scope :with_tax,       -> { where('spree_adjustments.included_tax <> 0') }
+    scope :without_tax,    -> { where('spree_adjustments.included_tax = 0') }
+    scope :payment_fee,    -> { where(AdjustmentScopes::PAYMENT_FEE_SCOPE) }
+    scope :shipping,       -> { where(AdjustmentScopes::SHIPPING_SCOPE) }
+    scope :eligible,       -> { where(AdjustmentScopes::ELIGIBLE_SCOPE) }
+
+    localize_number :amount
+
+    # Update the boolean _eligible_ attribute which determines which adjustments
+    # count towards the order's adjustment_total.
+    def set_eligibility
+      result = mandatory || (amount != 0 && eligible_for_originator?)
+      update_column(:eligible, result)
+    end
+
+    # Allow originator of the adjustment to perform an additional eligibility of the adjustment
+    # Should return _true_ if originator is absent or doesn't implement _eligible?_
+    def eligible_for_originator?
+      return true if originator.nil?
+
+      !originator.respond_to?(:eligible?) || originator.eligible?(source)
+    end
+
+    # Update both the eligibility and amount of the adjustment. Adjustments
+    # delegate updating of amount to their Originator when present, but only if
+    # +locked+ is false. Adjustments that are +locked+ will never change their amount.
+    #
+    # Adjustments delegate updating of amount to their Originator when present,
+    # but only if when they're in "open" state, closed or finalized adjustments
+    # are not recalculated.
+    #
+    # It receives +calculable+ as the updated source here so calculations can be
+    # performed on the current values of that source. If we used +source+ it
+    # could load the old record from db for the association. e.g. when updating
+    # more than on line items at once via accepted_nested_attributes the order
+    # object on the association would be in a old state and therefore the
+    # adjustment calculations would not performed on proper values
+    def update!(calculable = nil)
+      return if immutable?
+
+      # Fix for Spree issue #3381
+      # If we attempt to call 'source' before the reload, then source is currently
+      # the order object. After calling a reload, the source is the Shipment.
+      reload
+      originator.update_adjustment(self, calculable || source) if originator.present?
+      set_eligibility
+    end
+
+    def currency
+      adjustable ? adjustable.currency : Spree::Config[:currency]
+    end
+
+    def display_amount
+      Spree::Money.new(amount, currency: currency)
+    end
+
+    def immutable?
+      state != "open"
+    end
+
+    def set_included_tax!(rate)
+      tax = amount - (amount / (1 + rate))
+      set_absolute_included_tax! tax
+    end
+
+    def set_absolute_included_tax!(tax)
+      # This rubocop issue can now fixed by renaming Adjustment#update! to something else,
+      #   then AR's update! can be used instead of update_attributes!
+      # rubocop:disable Rails/ActiveRecordAliases
+      update_attributes! included_tax: tax.round(2)
+      # rubocop:enable Rails/ActiveRecordAliases
+    end
+
+    def display_included_tax
+      Spree::Money.new(included_tax, currency: currency)
+    end
+
+    def has_tax?
+      included_tax.positive?
+    end
+
+    def self.without_callbacks
+      skip_callback :save, :after, :update_adjustable
+      skip_callback :destroy, :after, :update_adjustable
+
+      result = yield
+    ensure
+      set_callback :save, :after, :update_adjustable
+      set_callback :destroy, :after, :update_adjustable
+
+      result
+    end
+
+    private
+
+    def update_adjustable
+      adjustable.update! if adjustable.is_a? Order
+    end
+  end
+end
diff --git a/app/models/spree/adjustment_decorator.rb b/app/models/spree/adjustment_decorator.rb
deleted file mode 100644
index cba3b3e879..0000000000
--- a/app/models/spree/adjustment_decorator.rb
+++ /dev/null
@@ -1,62 +0,0 @@
-require 'spree/localized_number'
-require 'concerns/adjustment_scopes'
-
-module Spree
-  Adjustment.class_eval do
-    extend Spree::LocalizedNumber
-
-    # Deletion of metadata is handled in the database.
-    # So we don't need the option `dependent: :destroy` as long as
-    # AdjustmentMetadata has no destroy logic itself.
-    has_one :metadata, class_name: 'AdjustmentMetadata'
-    belongs_to :tax_rate, -> { where spree_adjustments: { originator_type: 'Spree::TaxRate' } },
-               foreign_key: 'originator_id'
-
-    scope :enterprise_fee, -> { where(originator_type: 'EnterpriseFee') }
-    scope :admin,          -> { where(source_type: nil, originator_type: nil) }
-    scope :included_tax,   -> {
-      where(originator_type: 'Spree::TaxRate', adjustable_type: 'Spree::LineItem')
-    }
-
-    scope :with_tax,       -> { where('spree_adjustments.included_tax <> 0') }
-    scope :without_tax,    -> { where('spree_adjustments.included_tax = 0') }
-    scope :payment_fee,    -> { where(AdjustmentScopes::PAYMENT_FEE_SCOPE) }
-    scope :shipping,       -> { where(AdjustmentScopes::SHIPPING_SCOPE) }
-    scope :eligible,       -> { where(AdjustmentScopes::ELIGIBLE_SCOPE) }
-
-    localize_number :amount
-
-    def set_included_tax!(rate)
-      tax = amount - (amount / (1 + rate))
-      set_absolute_included_tax! tax
-    end
-
-    def set_absolute_included_tax!(tax)
-      # This rubocop issue can only be fixed when Adjustment#update! is brought from Spree to OFN
-      #   and renamed to something else, then AR's update! can be used instead of update_attributes!
-      # rubocop:disable Rails/ActiveRecordAliases
-      update_attributes! included_tax: tax.round(2)
-      # rubocop:enable Rails/ActiveRecordAliases
-    end
-
-    def display_included_tax
-      Spree::Money.new(included_tax, currency: currency)
-    end
-
-    def has_tax?
-      included_tax > 0
-    end
-
-    def self.without_callbacks
-      skip_callback :save, :after, :update_adjustable
-      skip_callback :destroy, :after, :update_adjustable
-
-      result = yield
-    ensure
-      set_callback :save, :after, :update_adjustable
-      set_callback :destroy, :after, :update_adjustable
-
-      result
-    end
-  end
-end
diff --git a/app/models/spree/calculator.rb b/app/models/spree/calculator.rb
new file mode 100644
index 0000000000..40bea838f5
--- /dev/null
+++ b/app/models/spree/calculator.rb
@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+module Spree
+  class Calculator < ActiveRecord::Base
+    belongs_to :calculable, polymorphic: true
+
+    # This method must be overriden in concrete calculator.
+    #
+    # It should return amount computed based on #calculable and/or optional parameter
+    def compute(_something = nil)
+      raise NotImplementedError, 'please use concrete calculator'
+    end
+
+    # overwrite to provide description for your calculators
+    def self.description
+      'Base Calculator'
+    end
+
+    ###################################################################
+
+    def self.register(*klasses); end
+
+    # Returns all calculators applicable for kind of work
+    def self.calculators
+      Rails.application.config.spree.calculators
+    end
+
+    def to_s
+      self.class.name.titleize.gsub("Calculator\/", "")
+    end
+
+    def description
+      self.class.description
+    end
+
+    def available?(_object)
+      true
+    end
+
+    private
+
+    # Given an object which might be an Order or a LineItem (amongst
+    # others), return a collection of line items.
+    def line_items_for(object)
+      if object.is_a?(Spree::LineItem)
+        [object]
+      elsif object.respond_to? :line_items
+        object.line_items
+      elsif object.respond_to?(:order) && object.order.present?
+        object.order.line_items
+      else
+        [object]
+      end
+    end
+  end
+end
diff --git a/app/models/spree/calculator_decorator.rb b/app/models/spree/calculator_decorator.rb
deleted file mode 100644
index 5d130d86aa..0000000000
--- a/app/models/spree/calculator_decorator.rb
+++ /dev/null
@@ -1,19 +0,0 @@
-module Spree
-  Calculator.class_eval do
-    private
-
-    # Given an object which might be an Order or a LineItem (amongst
-    # others), return a collection of line items.
-    def line_items_for(object)
-      if object.is_a?(Spree::LineItem)
-        [object]
-      elsif object.respond_to? :line_items
-        object.line_items
-      elsif object.respond_to?(:order) && object.order.present?
-        object.order.line_items
-      else
-        [object]
-      end
-    end
-  end
-end
diff --git a/app/models/spree/payment/processing.rb b/app/models/spree/payment/processing.rb
index 2579f76de7..c5a9979ee3 100644
--- a/app/models/spree/payment/processing.rb
+++ b/app/models/spree/payment/processing.rb
@@ -179,11 +179,11 @@ module Spree
                     # For more information, please see Spree::Payment#set_unique_identifier
                     order_id: gateway_order_id }
 
-        options.merge!({ shipping: order.ship_total * 100,
-                         tax: order.tax_total * 100,
-                         subtotal: order.item_total * 100,
-                         discount: order.promo_total * 100,
-                         currency: currency })
+        options.merge!(shipping: order.ship_total * 100,
+                       tax: order.tax_total * 100,
+                       subtotal: order.item_total * 100,
+                       discount: 0,
+                       currency: currency)
 
         options.merge!({ billing_address: order.bill_address.try(:active_merchant_hash),
                          shipping_address: order.ship_address.try(:active_merchant_hash) })
diff --git a/app/models/spree/tax_category.rb b/app/models/spree/tax_category.rb
new file mode 100644
index 0000000000..a856e06e10
--- /dev/null
+++ b/app/models/spree/tax_category.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module Spree
+  class TaxCategory < ActiveRecord::Base
+    acts_as_paranoid
+    validates :name, presence: true, uniqueness: { scope: :deleted_at }
+
+    has_many :tax_rates, dependent: :destroy
+
+    before_save :set_default_category
+
+    def set_default_category
+      # set existing default tax category to false if this one has been marked as default
+
+      return unless is_default && tax_category = self.class.find_by(is_default: true)
+
+      tax_category.update_column(:is_default, false) unless tax_category == self
+    end
+  end
+end
diff --git a/app/models/spree/tax_rate.rb b/app/models/spree/tax_rate.rb
new file mode 100644
index 0000000000..0e25c59b63
--- /dev/null
+++ b/app/models/spree/tax_rate.rb
@@ -0,0 +1,132 @@
+# frozen_string_literal: false
+
+module Spree
+  class DefaultTaxZoneValidator < ActiveModel::Validator
+    def validate(record)
+      return unless record.included_in_price
+
+      return if Zone.default_tax
+
+      record.errors.add(:included_in_price, Spree.t(:included_price_validation))
+    end
+  end
+end
+
+module Spree
+  class TaxRate < ActiveRecord::Base
+    acts_as_paranoid
+    include Spree::Core::CalculatedAdjustments
+    belongs_to :zone, class_name: "Spree::Zone"
+    belongs_to :tax_category, class_name: "Spree::TaxCategory"
+
+    validates :amount, presence: true, numericality: true
+    validates :tax_category_id, presence: true
+    validates_with DefaultTaxZoneValidator
+
+    scope :by_zone, ->(zone) { where(zone_id: zone) }
+
+    # Gets the array of TaxRates appropriate for the specified order
+    def self.match(order)
+      return [] if order.distributor && !order.distributor.charges_sales_tax
+      return [] unless order.tax_zone
+
+      all.select do |rate|
+        rate.zone == order.tax_zone || rate.zone.contains?(order.tax_zone) || rate.zone.default_tax
+      end
+    end
+
+    def self.adjust(order)
+      order.adjustments.tax.destroy_all
+      order.line_item_adjustments.where(originator_type: 'Spree::TaxRate').destroy_all
+
+      match(order).each do |rate|
+        rate.adjust(order)
+      end
+    end
+
+    # For VAT, the default rate is the rate that is configured for the default category
+    # It is needed for every price calculation (as all customer facing prices include VAT)
+    # Here we return the actual amount, which may be 0 in case of wrong setup, but is never nil
+    def self.default
+      category = TaxCategory.includes(:tax_rates).find_by(is_default: true)
+      return 0 unless category
+
+      address ||= Address.new(country_id: Spree::Config[:default_country_id])
+      rate = category.tax_rates.detect { |tax_rate| tax_rate.zone.include? address }.try(:amount)
+
+      rate || 0
+    end
+
+    # Creates necessary tax adjustments for the order.
+    def adjust(order)
+      label = create_label
+      if included_in_price
+        if Zone.default_tax.contains? order.tax_zone
+          order.line_items.each { |line_item| create_adjustment(label, line_item, line_item) }
+        else
+          amount = -1 * calculator.compute(order)
+          label = Spree.t(:refund) + label
+
+          order.adjustments.create(
+            amount: amount,
+            source: order,
+            originator: self,
+            state: "closed",
+            label: label
+          )
+        end
+      else
+        create_adjustment(label, order, order)
+      end
+
+      order.adjustments(:reload)
+      order.line_items(:reload)
+      # TaxRate adjustments (order.adjustments.tax)
+      #   and price adjustments (tax included on line items) consist of 100% tax
+      (order.adjustments.tax + order.price_adjustments).each do |adjustment|
+        adjustment.set_absolute_included_tax! adjustment.amount
+      end
+    end
+
+    # Manually apply a TaxRate to a particular amount. TaxRates normally compute against
+    # LineItems or Orders, so we mock out a line item here to fit the interface
+    # that our calculator (usually DefaultTax) expects.
+    def compute_tax(amount)
+      line_item = LineItem.new quantity: 1
+      line_item.tax_category = tax_category
+      line_item.define_singleton_method(:price) { amount }
+
+      # Tax on adjustments (represented by the included_tax field) is always inclusive of
+      # tax. However, there's nothing to stop an admin from setting one up with a tax rate
+      # that's marked as not inclusive of tax, and that would result in the DefaultTax
+      # calculator generating a slightly incorrect value. Therefore, we treat the tax
+      # rate as inclusive of tax for the calculations below, regardless of its original
+      # setting.
+      with_tax_included_in_price do
+        calculator.compute line_item
+      end
+    end
+
+    private
+
+    def create_label
+      label = ""
+      label << (name.presence || tax_category.name) + " "
+      label << (show_rate_in_label? ? "#{amount * 100}%" : "")
+    end
+
+    def with_tax_included_in_price
+      old_included_in_price = included_in_price
+
+      self.included_in_price = true
+      calculator.calculable.included_in_price = true
+
+      result = yield
+    ensure
+      self.included_in_price = old_included_in_price
+      calculator.calculable.included_in_price = old_included_in_price
+
+      result
+    end
+  end
+end
diff --git a/app/models/spree/tax_rate_decorator.rb b/app/models/spree/tax_rate_decorator.rb
deleted file mode 100644
index f15ded6de8..0000000000
--- a/app/models/spree/tax_rate_decorator.rb
+++ /dev/null
@@ -1,61 +0,0 @@
-module Spree
-  TaxRate.class_eval do
-    class << self
-      def match(order)
-        return [] if order.distributor && !order.distributor.charges_sales_tax
-        return [] unless order.tax_zone
-
-        all.select do |rate|
-          rate.zone == order.tax_zone || rate.zone.contains?(order.tax_zone) || rate.zone.default_tax
-        end
-      end
-    end
-
-    def adjust_with_included_tax(order)
-      adjust_without_included_tax(order)
-
-      order.adjustments(:reload)
-      order.line_items(:reload)
-      # TaxRate adjustments (order.adjustments.tax) and price adjustments (tax included on line items) consist of 100% tax
-      (order.adjustments.tax + order.price_adjustments).each do |adjustment|
-        adjustment.set_absolute_included_tax! adjustment.amount
-      end
-    end
-    alias_method_chain :adjust, :included_tax
-
-    # Manually apply a TaxRate to a particular amount. TaxRates normally compute against
-    # LineItems or Orders, so we mock out a line item here to fit the interface
-    # that our calculator (usually DefaultTax) expects.
-    def compute_tax(amount)
-      line_item = LineItem.new quantity: 1
-      line_item.tax_category = tax_category
-      line_item.define_singleton_method(:price) { amount }
-
-      # Tax on adjustments (represented by the included_tax field) is always inclusive of
-      # tax. However, there's nothing to stop an admin from setting one up with a tax rate
-      # that's marked as not inclusive of tax, and that would result in the DefaultTax
-      # calculator generating a slightly incorrect value. Therefore, we treat the tax
-      # rate as inclusive of tax for the calculations below, regardless of its original
-      # setting.
-      with_tax_included_in_price do
-        calculator.compute line_item
-      end
-    end
-
-    private
-
-    def with_tax_included_in_price
-      old_included_in_price = included_in_price
-
-      self.included_in_price = true
-      calculator.calculable.included_in_price = true
-
-      result = yield
-    ensure
-      self.included_in_price = old_included_in_price
-      calculator.calculable.included_in_price = old_included_in_price
-
-      result
-    end
-  end
-end
diff --git a/spec/factories/return_authorization_factory.rb b/spec/factories/return_authorization_factory.rb
new file mode 100644
index 0000000000..ad77351a40
--- /dev/null
+++ b/spec/factories/return_authorization_factory.rb
@@ -0,0 +1,9 @@
+FactoryBot.define do
+  factory :return_authorization, class: Spree::ReturnAuthorization do
+    number '100'
+    amount 100.00
+    association(:order, factory: :shipped_order)
+    reason 'no particular reason'
+    state 'received'
+  end
+end
diff --git a/spec/models/spree/adjustment_spec.rb b/spec/models/spree/adjustment_spec.rb
index 0ea3eea273..ef66733ec1 100644
--- a/spec/models/spree/adjustment_spec.rb
+++ b/spec/models/spree/adjustment_spec.rb
@@ -2,6 +2,212 @@ require 'spec_helper'
 
 module Spree
   describe Adjustment do
+    let(:order) { build(:order) }
+    let(:adjustment) { Spree::Adjustment.create(label: "Adjustment", amount: 5) }
+
+    describe "scopes" do
+      let!(:arbitrary_adjustment) { create(:adjustment, source: nil, label: "Arbitrary") }
+      let!(:return_authorization_adjustment) { create(:adjustment, source: create(:return_authorization)) }
+
+      it "returns return_authorization adjustments" do
+        expect(Spree::Adjustment.return_authorization.to_a).to eq [return_authorization_adjustment]
+      end
+    end
+
+    context "#update!" do
+      context "when originator present" do
+        let(:originator) { double("originator", update_adjustment: nil) }
+        before do
+          allow(originator).to receive_messages update_amount: true
+          allow(adjustment).to receive_messages originator: originator, label: 'adjustment', amount: 0
+        end
+
+        it "should do nothing when closed" do
+          adjustment.close
+          expect(originator).not_to receive(:update_adjustment)
+          adjustment.update!
+        end
+
+        it "should do nothing when finalized" do
+          adjustment.finalize
+          expect(originator).not_to receive(:update_adjustment)
+          adjustment.update!
+        end
+
+        it "should set the eligibility" do
+          expect(adjustment).to receive(:set_eligibility)
+          adjustment.update!
+        end
+
+        it "should ask the originator to update_adjustment" do
+          expect(originator).to receive(:update_adjustment)
+          adjustment.update!
+        end
+      end
+
+      it "should do nothing when originator is nil" do
+        allow(adjustment).to receive_messages originator: nil
+        expect(adjustment).not_to receive(:amount=)
+        adjustment.update!
+      end
+    end
+
+    context "#eligible? after #set_eligibility" do
+      context "when amount is 0" do
+        before { adjustment.amount = 0 }
+        it "should be eligible if mandatory?" do
+          adjustment.mandatory = true
+          adjustment.set_eligibility
+          expect(adjustment).to be_eligible
+        end
+
+        it "should not be eligible unless mandatory?" do
+          adjustment.mandatory = false
+          adjustment.set_eligibility
+          expect(adjustment).to_not be_eligible
+        end
+      end
+
+      context "when amount is greater than 0" do
+        before { adjustment.amount = 25.00 }
+
+        it "should be eligible if mandatory?" do
+          adjustment.mandatory = true
+          adjustment.set_eligibility
+          expect(adjustment).to be_eligible
+        end
+
+        it "should be eligible if not mandatory and eligible for the originator" do
+          adjustment.mandatory = false
+          allow(adjustment).to receive_messages(eligible_for_originator?: true)
+          adjustment.set_eligibility
+          expect(adjustment).to be_eligible
+        end
+
+        it "should not be eligible if not mandatory not eligible for the originator" do
+          adjustment.mandatory = false
+          allow(adjustment).to receive_messages(eligible_for_originator?: false)
+          adjustment.set_eligibility
+          expect(adjustment).to_not be_eligible
+        end
+      end
+    end
+
+    context "#save" do
+      it "should call order#update!" do
+        adjustment = Spree::Adjustment.new(
+          adjustable: order,
+          amount: 10,
+          label: "Foo"
+        )
+        expect(order).to receive(:update!)
+        adjustment.save
+      end
+    end
+
+    context "adjustment state" do
+      let(:adjustment) { create(:adjustment, state: 'open') }
+
+      context "#immutable?" do
+        it "is true when adjustment state isn't open" do
+          adjustment.state = "closed"
+          expect(adjustment).to be_immutable
+          adjustment.state = "finalized"
+          expect(adjustment).to be_immutable
+        end
+
+        it "is false when adjustment state is open" do
+          adjustment.state = "open"
+          expect(adjustment).to_not be_immutable
+        end
+      end
+
+      context "#finalized?" do
+        it "is true when adjustment state is finalized" do
+          adjustment.state = "finalized"
+          expect(adjustment).to be_finalized
+        end
+
+        it "is false when adjustment state isn't finalized" do
+          adjustment.state = "closed"
+          expect(adjustment).to_not be_finalized
+          adjustment.state = "open"
+          expect(adjustment).to_not be_finalized
+        end
+      end
+    end
+
+    context "#eligible_for_originator?" do
+      context "with no originator" do
+        specify { expect(adjustment).to be_eligible_for_originator }
+      end
+
+      context "with originator that doesn't have 'eligible?'" do
+        before { adjustment.originator = build(:tax_rate) }
+        specify { expect(adjustment).to be_eligible_for_originator }
+      end
+
+      context "with originator that has 'eligible?'" do
+        let(:originator) { Spree::TaxRate.new }
+        before { adjustment.originator = originator }
+
+        context "and originator is eligible for order" do
+          before { allow(originator).to receive_messages(eligible?: true) }
+          specify { expect(adjustment).to be_eligible_for_originator }
+        end
+
+        context "and originator is not eligible for order" do
+          before { allow(originator).to receive_messages(eligible?: false) }
+          specify { expect(adjustment).to_not be_eligible_for_originator }
+        end
+      end
+    end
+
+    context "#display_amount" do
+      before { adjustment.amount = 10.55 }
+
+      context "with display_currency set to true" do
+        before { Spree::Config[:display_currency] = true }
+
+        it "shows the currency" do
+          expect(adjustment.display_amount.to_s).to eq "$10.55 #{Spree::Config[:currency]}"
+        end
+      end
+
+      context "with display_currency set to false" do
+        before { Spree::Config[:display_currency] = false }
+
+        it "does not include the currency" do
+          expect(adjustment.display_amount.to_s).to eq "$10.55"
+        end
+      end
+
+      context "with currency set to JPY" do
+        context "when adjustable is set to an order" do
+          before do
+            allow(order).to receive(:currency) { 'JPY' }
+            adjustment.adjustable = order
+          end
+
+          it "displays in JPY" do
+            expect(adjustment.display_amount.to_s).to eq "¥11"
+          end
+        end
+
+        context "when adjustable is nil" do
+          it "displays in the default currency" do
+            expect(adjustment.display_amount.to_s).to eq "$10.55"
+          end
+        end
+      end
+    end
+
+    context '#currency' do
+      it 'returns the globally configured currency' do
+        expect(adjustment.currency).to eq Spree::Config[:currency]
+      end
+    end
+
     it "has metadata" do
       adjustment = create(:adjustment, metadata: create(:adjustment_metadata))
       expect(adjustment.metadata).to be
@@ -39,7 +245,7 @@ module Spree
         let!(:zone)        { create(:zone_with_member) }
         let!(:order)       { create(:order, bill_address: create(:address)) }
         let!(:line_item)   { create(:line_item, order: order) }
-        let(:tax_rate)     { create(:tax_rate, included_in_price: true, calculator: Calculator::FlatRate.new(preferred_amount: 0.1)) }
+        let(:tax_rate)     { create(:tax_rate, included_in_price: true, calculator: ::Calculator::FlatRate.new(preferred_amount: 0.1)) }
         let(:adjustment)   { line_item.adjustments(:reload).first }
 
         before do
@@ -205,8 +411,10 @@ module Spree
         end
 
         context "when enterprise fees inherit their tax_category from the product they are applied to" do
-          let(:product_tax_rate)             { create(:tax_rate, included_in_price: true, calculator: ::Calculator::DefaultTax.new, zone: zone, amount: 0.2) }
-          let(:product_tax_category)         { create(:tax_category, tax_rates: [product_tax_rate]) }
+          let(:product_tax_rate) {
+            create(:tax_rate, included_in_price: true, calculator: ::Calculator::DefaultTax.new, zone: zone, amount: 0.2)
+          }
+          let(:product_tax_category) { create(:tax_category, tax_rates: [product_tax_rate]) }
 
           before do
             variant.product.update_attribute(:tax_category_id, product_tax_category.id)
diff --git a/spec/models/spree/tax_category_spec.rb b/spec/models/spree/tax_category_spec.rb
new file mode 100644
index 0000000000..77e9a02501
--- /dev/null
+++ b/spec/models/spree/tax_category_spec.rb
@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Spree::TaxCategory do
+  context 'default tax category' do
+    let(:tax_category) { create(:tax_category) }
+    let(:new_tax_category) { create(:tax_category) }
+
+    before do
+      tax_category.update_column(:is_default, true)
+    end
+
+    it "should undefault the previous default tax category" do
+      new_tax_category.update({ is_default: true })
+      expect(new_tax_category.is_default).to be_truthy
+
+      tax_category.reload
+      expect(tax_category.is_default).to be_falsy
+    end
+
+    it "undefaults the previous default tax category
+      except when updating the existing default tax category" do
+      tax_category.update_column(:description, "Updated description")
+
+      tax_category.reload
+      expect(tax_category.is_default).to be_truthy
+    end
+  end
+end