Hi,
I am a PHP developer with 7+ years of experience.
From your requirement, what I have understood is :
You already have a checkout script with 2 payment gateways integrated - Paypal and 2 Checkout.
Now you need to add coupon system to the checkout process so that your users can avail discounts while checkout, right ?
I can build this solution for you very easily. I have already built so many ecommerce scripts, and I have the expertise in coupon management.
I will build this solution with 24 hours, if all the requirements are clear.
Looking forward to work with you today :)
Thanks.