Exclude line item additional taxes from unit cancel adjustment amount

core/app/models/spree/unit_cancel.rb

@@ -34,6 +34,17 @@ def adjust!
   # This method is used by Adjustment#update to recalculate the cost.
   def compute_amount(line_item)
     raise "Adjustable does not match line item" unless line_item == inventory_unit.line_item
-    -(line_item.total.to_d / line_item.inventory_units.not_canceled.reject(&:original_return_item ).size)
+
+    -weighted_line_item_amount(line_item)
+  end
+
+  private
+
+  def weighted_line_item_amount(line_item)
+    line_item.total_before_tax / quantity_of_line_item(line_item)
+  end
+
+  def quantity_of_line_item(line_item)
+    BigDecimal(line_item.inventory_units.not_canceled.reject(&:original_return_item).size)
   end
 end

core/spec/models/spree/unit_cancel_spec.rb

@@ -76,5 +76,46 @@
         expect(line_item.reload.total.to_d).to eq(0)
       end
     end
+
+    context 'when line item has additional taxes' do
+      let(:world_zone) { create(:zone, :with_country) }
+      let(:tax_category) { create :tax_category }
+      let(:product) { create :product, tax_category: tax_category }
+      let!(:additional_tax_rate) do
+        create(
+          :tax_rate,
+          name: 'Additional tax',
+          tax_categories: [tax_category],
+          zone: world_zone,
+          included_in_price: false,
+          amount: 0.15
+        )
+      end
+      let(:shipping_category) { create :shipping_category }
+      let(:shipping_method) do
+        create :shipping_method,
+               cost: 8.00,
+               shipping_categories: [shipping_category],
+               tax_category: tax_category,
+               zones: [world_zone]
+      end
+      let(:shipping_address) { create :address, country_iso_code: world_zone.countries.first.iso }
+      let(:order) do
+        create(
+          :order_with_line_items,
+          ship_address: shipping_address,
+          line_items_attributes: [{ product: product }]
+        )
+      end
+      let(:line_item) { order.line_items.first }
+      let(:inventory_unit) { line_item.inventory_units.first }
+
+      before { order.recalculate }
+
+      it 'does not include line item additional taxes' do
+        expect(line_item.additional_tax_total).not_to eq 0
+        expect(subject).to eq(-5.0)
+      end
+    end
   end
 end