Update pending payment for completed order

Update pending payment for cash order, so we can take into account any
changes affecting order toral (shipments, line item quantity etc...).
This is to allow payment capture to cover the order total, not just
the amount due at checkout.

engines/order_management/app/services/order_management/order/updater.rb

@@ -234,7 +234,10 @@ module OrderManagement
       end
 
       def update_pending_payment
-        return unless order.state.in? ["payment", "confirmation"]
+        # We only want to update complete order pending payment when it's a cash payment. We assume
+        # that if the payment was a credit card it would alread have been processed, so we don't
+        # bother checking the payment type
+        return unless order.state.in? ["payment", "confirmation", "complete"]
         return unless order.pending_payments.any?
 
         order.pending_payments.first.update_attribute :amount, order.total

engines/order_management/spec/services/order_management/order/updater_spec.rb

@@ -105,6 +105,20 @@ module OrderManagement
           expect(shipment).to receive(:update!).with(order)
           updater.update_shipments
         end
+
+        context "whith pending payments" do
+          let(:order) { create(:completed_order_with_totals) }
+
+          it "updates pending payments" do
+            payment = create(:payment, order: , amount: order.total)
+
+            # update order so the order total will change
+            update_order_quantity(order)
+            order.payments.reload
+
+            expect { updater.update }.to change { payment.reload.amount }.from(50).to(60)
+          end
+        end
       end
 
       context "incompleted order" do