Fix ECE modal not loading on pay for order page when coupon is applied

[Mayisha]

When an order uses a coupon, the express payment modal does not load on the pay for order page. The issue happens because we are not sending the coupon as a line item when loading the modal on the pay for order page. This results in a mismatch in the total.

Changes proposed in this Pull Request:

• Adding the coupon amount as a line item.

Testing instructions

• Enable the new checkout experience.
• Enable the ECE feature flag.
• Enable the express payment methods on Stripe settings page (Google Pay/Apple Pay and Link).
• As an admin, go to the order creation page.
• Add a product to the order and add a coupon.
• Add a customer and create the order.
• Open the 'Customer payment page →' link as the customer.
• Notice that the express checkout buttons are loading fine.
• Now in develop branch, click on the Link or Google Pay button.
• Notice that the corresponding modal never loads.

• In this branch, the modals should load correctly.
• Confirm that the amounts (subtotal, coupon, total) displayed on the modal are correct.
• Confirm that the final charged amount is correct.

[wjrosa]

Thanks for working on this fix, Mayisha! Code is good and it works as intended: