Sum Spree::ShippingManifest::ManifestItem.item_cost

This commit is going to add `item_cost` calculation, all these item costs will be summed.

core/app/models/spree/shipment.rb

@@ -184,7 +184,7 @@ def inventory_units_for_item(line_item, variant = nil)
     end
 
     def item_cost
-      line_items.map(&:total).sum
+      manifest.sum(&:item_cost)
     end
 
     def ready_or_pending?

core/app/models/spree/shipping_manifest.rb

@@ -1,7 +1,11 @@
 # frozen_string_literal: true
 
 class Spree::ShippingManifest
-  ManifestItem = Struct.new(:line_item, :variant, :quantity, :states)
+  ManifestItem = Struct.new(:line_item, :variant, :quantity, :states) do
+    def item_cost
+      (line_item.price + (line_item.adjustment_total / line_item.quantity)) * quantity
+    end
+  end
 
   def initialize(inventory_units:)
     @inventory_units = inventory_units.to_a

core/spec/models/spree/shipment_spec.rb

@@ -141,6 +141,26 @@
     it 'should equal line items final amount with tax' do
       expect(shipment.item_cost).to eql(11.0)
     end
+
+    context 'with more shipments for the same line_item' do
+      let(:order) do
+        create(
+          :order_with_line_items,
+          line_items_attributes: [{ price: 10, variant: variant, quantity: 2 }],
+          ship_address: ship_address,
+        )
+      end
+      let(:other_shipment) { order.shipments.create(stock_location_id: shipment.stock_location) }
+
+      it 'returns shipment line items amount with tax' do
+        expect(order.shipments.first.item_cost).to eql(22.0)
+        expect {
+          order.inventory_units.last.update(shipment_id: other_shipment.id)
+        }.to change { order.shipments.reload.first.item_cost }.from(22.0).to(11.0)
+
+        expect(order.shipments.second.item_cost).to eql(11.0)
+      end
+    end
   end
 
   it "#discounted_cost" do