diff --git a/app/models/calculator/weight.rb b/app/models/calculator/weight.rb
index ae65026a46..410a53b2f8 100644
--- a/app/models/calculator/weight.rb
+++ b/app/models/calculator/weight.rb
@@ -12,19 +12,5 @@ module Calculator
       total_weight = line_items.sum { |li| ((li.variant.andand.weight || 0) * li.quantity) }
       total_weight * preferred_per_kg
     end
-
-    private
-
-    def line_items_for(object)
-      if object.respond_to? :order
-        object.order.line_items
-      elsif object.respond_to? :line_items
-        object.line_items
-      elsif object.respond_to?(:variant) && object.respond_to?(:quantity)
-        [object]
-      else
-        raise "Unknown object type: #{object.inspect}"
-      end
-    end
   end
 end
diff --git a/app/models/spree/calculator_decorator.rb b/app/models/spree/calculator_decorator.rb
index f8be4d00ef..29db0a5c0d 100644
--- a/app/models/spree/calculator_decorator.rb
+++ b/app/models/spree/calculator_decorator.rb
@@ -1,18 +1,20 @@
 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.respond_to? :line_items
+      if object.is_a?(Spree::LineItem)
+        [object]
+      elsif object.respond_to? :line_items
         object.line_items
+      elsif object.order.present?
+        object.order.line_items
       else
         [object]
       end
     end
-
   end
 end
diff --git a/spec/lib/open_food_network/enterprise_fee_calculator_spec.rb b/spec/lib/open_food_network/enterprise_fee_calculator_spec.rb
index 01b97346f7..ae79303b8d 100644
--- a/spec/lib/open_food_network/enterprise_fee_calculator_spec.rb
+++ b/spec/lib/open_food_network/enterprise_fee_calculator_spec.rb
@@ -1,3 +1,4 @@
+require 'spec_helper'
 require 'open_food_network/enterprise_fee_calculator'
 
 module OpenFoodNetwork
diff --git a/spec/models/calculator/weight_spec.rb b/spec/models/calculator/weight_spec.rb
index dee2f3d067..c45da8850a 100644
--- a/spec/models/calculator/weight_spec.rb
+++ b/spec/models/calculator/weight_spec.rb
@@ -2,13 +2,13 @@ require 'spec_helper'
 
 describe Calculator::Weight do
   it "computes shipping cost for an order by total weight" do
-    variant1 = double(:variant, weight: 10)
-    variant2 = double(:variant, weight: 20)
-    variant3 = double(:variant, weight: nil)
+    variant1 = build(:variant, weight: 10)
+    variant2 = build(:variant, weight: 20)
+    variant3 = build(:variant, weight: nil)
 
-    line_item1 = double(:line_item, variant: variant1, quantity: 1)
-    line_item2 = double(:line_item, variant: variant2, quantity: 3)
-    line_item3 = double(:line_item, variant: variant3, quantity: 5)
+    line_item1 = build(:line_item, variant: variant1, quantity: 1)
+    line_item2 = build(:line_item, variant: variant2, quantity: 3)
+    line_item3 = build(:line_item, variant: variant3, quantity: 5)
 
     order = double(:order, line_items: [line_item1, line_item2, line_item3])
 
@@ -17,20 +17,20 @@ describe Calculator::Weight do
   end
 
   it "computes shipping cost for a line item" do
-    variant = double(:variant, weight: 10)
+    variant = build(:variant, weight: 10)
 
-    line_item = double(:line_item, variant: variant, quantity: 2)
+    line_item = build(:line_item, variant: variant, quantity: 2)
 
     subject.set_preference(:per_kg, 10)
     expect(subject.compute(line_item)).to eq(10 * 2 * 10)
   end
 
   it "computes shipping cost for an object with an order" do
-    variant1 = double(:variant, weight: 10)
-    variant2 = double(:variant, weight: 5)
+    variant1 = build(:variant, weight: 10)
+    variant2 = build(:variant, weight: 5)
 
-    line_item1 = double(:line_item, variant: variant1, quantity: 1)
-    line_item2 = double(:line_item, variant: variant2, quantity: 2)
+    line_item1 = build(:line_item, variant: variant1, quantity: 1)
+    line_item2 = build(:line_item, variant: variant2, quantity: 2)
 
     order = double(:order, line_items: [line_item1, line_item2])
     object_with_order = double(:object_with_order, order: order)
diff --git a/spec/models/spree/calculator/flat_percent_item_total_spec.rb b/spec/models/spree/calculator/flat_percent_item_total_spec.rb
index 998b5fca2a..72fe29d9d1 100644
--- a/spec/models/spree/calculator/flat_percent_item_total_spec.rb
+++ b/spec/models/spree/calculator/flat_percent_item_total_spec.rb
@@ -2,7 +2,7 @@ require 'spec_helper'
 
 describe Spree::Calculator::FlatPercentItemTotal do
   let(:calculator) { Spree::Calculator::FlatPercentItemTotal.new }
-  let(:line_item) { instance_double(Spree::LineItem, amount: 10) }
+  let(:line_item) { build(:line_item, price: 10, quantity: 1) }
 
   before { calculator.stub :preferred_flat_percent => 10 }
 
diff --git a/spec/models/spree/calculator/flexi_rate_spec.rb b/spec/models/spree/calculator/flexi_rate_spec.rb
index 6659da86fd..b821f4fc74 100644
--- a/spec/models/spree/calculator/flexi_rate_spec.rb
+++ b/spec/models/spree/calculator/flexi_rate_spec.rb
@@ -1,7 +1,7 @@
 require 'spec_helper'
 
 describe Spree::Calculator::FlexiRate do
-  let(:line_item) { instance_double(Spree::LineItem, amount: 10, quantity: quantity) }
+  let(:line_item) { build(:line_item, quantity: quantity) }
   let(:calculator) do
     Spree::Calculator::FlexiRate.new(
       preferred_first_item: 2,
diff --git a/spec/models/spree/calculator/per_item_spec.rb b/spec/models/spree/calculator/per_item_spec.rb
index 7f483372d5..a50c5264b3 100644
--- a/spec/models/spree/calculator/per_item_spec.rb
+++ b/spec/models/spree/calculator/per_item_spec.rb
@@ -3,7 +3,7 @@ require 'spec_helper'
 describe Spree::Calculator::PerItem do
   let(:calculator) { Spree::Calculator::PerItem.new(preferred_amount: 10) }
   let(:shipping_calculable) { double(:calculable) }
-  let(:line_item) { double(:line_item, quantity: 5, product: double(:product)) }
+  let(:line_item) { build(:line_item, quantity: 5) }
 
   it "correctly calculates on a single line item object" do
     calculator.stub(calculable: shipping_calculable)