A coupon code is passed as a request parameter in url e.g. [url removed, login to view] .
Create table (coupon, discount %, expiry date, used date, user name) and minimal backend to create/edit/delete.
Implement this on your server for testing/demo.
Note: This project next phase will include, 1. send the coupon using mail
from admin (create/edit) screens. 2. mark the coupon as "used" by php
code (currently admin marks it), 3. paypal modifications to return success
or failure of transaction to mark coupon. etc. You are welcome to give a
separate quote for this phase.
More projects in line for the performing winner. Cost is a deciding factor.