diff --git a/app/models/spree/stock/availability_validator.rb b/app/models/spree/stock/availability_validator.rb
index fe9bc19442..1ca304e8d0 100644
--- a/app/models/spree/stock/availability_validator.rb
+++ b/app/models/spree/stock/availability_validator.rb
@@ -2,23 +2,51 @@ module Spree
   module Stock
     class AvailabilityValidator < ActiveModel::Validator
       def validate(line_item)
-        if shipment = line_item.target_shipment
-          units = shipment.inventory_units_for(line_item.variant)
-          return if units.count > line_item.quantity
-          quantity = line_item.quantity - units.count
-        else
-          quantity = line_item.quantity
-        end
+        # OFN specific check for in-memory :skip_stock_check attribute
+        return if line_item.skip_stock_check
 
-        quantifier = Stock::Quantifier.new(line_item.variant_id)
+        quantity_to_validate = line_item.quantity - quantity_in_shipment(line_item)
+        return if quantity_to_validate < 1
 
-        unless quantifier.can_supply? quantity
-          variant = line_item.variant
-          display_name = %Q{#{variant.name}}
-          display_name += %Q{ (#{variant.options_text})} unless variant.options_text.blank?
+        validate_quantity(line_item, quantity_to_validate)
+      end
 
-          line_item.errors[:quantity] << Spree.t(:out_of_stock, :scope => :order_populator, :item => display_name.inspect)
-        end
+      private
+
+      # This is an adapted version of a fix to the inventory_units not being considered here.
+      # See #3090 for details.
+      # This can be removed after upgrading to Spree 2.4.
+      def quantity_in_shipment(line_item)
+        shipment = line_item_shipment(line_item)
+        return 0 unless shipment
+
+        units = shipment.inventory_units_for(line_item.variant)
+        units.count
+      end
+
+      def line_item_shipment(line_item)
+        return line_item.target_shipment if line_item.target_shipment
+        return line_item.order.shipments.first if line_item.order.andand.shipments.any?
+      end
+
+      # Overrides Spree v2.0.4 validate method version to:
+      #   - scope variants to hub and thus acivate variant overrides
+      #   - use calculated quantity instead of the line_item.quantity
+      #   - rely on Variant.can_supply? instead of Stock::Quantified.can_supply?
+      #       so that it works correctly for variant overrides
+      def validate_quantity(line_item, quantity)
+        line_item.scoper.scope(line_item.variant)
+
+        add_out_of_stock_error(line_item) unless line_item.variant.can_supply? quantity
+      end
+
+      def add_out_of_stock_error(line_item)
+        variant = line_item.variant
+        display_name = variant.name.to_s
+        display_name += %{(#{variant.options_text})} if variant.options_text.present?
+        line_item.errors[:quantity] << Spree.t(:out_of_stock,
+                                               scope: :order_populator,
+                                               item: display_name.inspect)
       end
     end
   end
diff --git a/app/models/spree/stock/availability_validator_decorator.rb b/app/models/spree/stock/availability_validator_decorator.rb
deleted file mode 100644
index c3ee4c4818..0000000000
--- a/app/models/spree/stock/availability_validator_decorator.rb
+++ /dev/null
@@ -1,49 +0,0 @@
-Spree::Stock::AvailabilityValidator.class_eval do
-  def validate(line_item)
-    # OFN specific check for in-memory :skip_stock_check attribute
-    return if line_item.skip_stock_check
-
-    quantity_to_validate = line_item.quantity - quantity_in_shipment(line_item)
-    return if quantity_to_validate < 1
-
-    validate_quantity(line_item, quantity_to_validate)
-  end
-
-  private
-
-  # This is an adapted version of a fix to the inventory_units not being considered here.
-  # See #3090 for details.
-  # This can be removed after upgrading to Spree 2.4.
-  def quantity_in_shipment(line_item)
-    shipment = line_item_shipment(line_item)
-    return 0 unless shipment
-
-    units = shipment.inventory_units_for(line_item.variant)
-    units.count
-  end
-
-  def line_item_shipment(line_item)
-    return line_item.target_shipment if line_item.target_shipment
-    return line_item.order.shipments.first if line_item.order.andand.shipments.any?
-  end
-
-  # Overrides Spree v2.0.4 validate method version to:
-  #   - scope variants to hub and thus acivate variant overrides
-  #   - use calculated quantity instead of the line_item.quantity
-  #   - rely on Variant.can_supply? instead of Stock::Quantified.can_supply?
-  #       so that it works correctly for variant overrides
-  def validate_quantity(line_item, quantity)
-    line_item.scoper.scope(line_item.variant)
-
-    add_out_of_stock_error(line_item) unless line_item.variant.can_supply? quantity
-  end
-
-  def add_out_of_stock_error(line_item)
-    variant = line_item.variant
-    display_name = variant.name.to_s
-    display_name += %{(#{variant.options_text})} if variant.options_text.present?
-    line_item.errors[:quantity] << Spree.t(:out_of_stock,
-                                           scope: :order_populator,
-                                           item: display_name.inspect)
-  end
-end