Add Coupon Field to Payment Gateway, few small tweaks. Developer sick today, need immediate fix

We need a developer to add a coupon field to our payment gateway and make a few small tweaks to the payment page. The work includes updating the checkout experience, handling coupon validation, and ensuring the changes work smoothly with our existing payment flow. Please review the attached document for full details and let us know if you have questions. We’re looking for someone who can complete this efficiently and accurately.