Fix item_cost for split shipments

core/app/models/spree/shipment.rb

@@ -133,7 +133,7 @@ def inventory_units_for_item(line_item, variant = nil)
     end
 
     def item_cost
-      line_items.sum(&:total)
+      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.total * quantity / line_item.quantity
+    end
+  end
 
   attr_reader :inventory_units, :order
 

core/spec/models/spree/shipment_spec.rb

@@ -210,6 +210,26 @@
     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: 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
 
   describe "#total_before_tax" do