Discount Coupon Mod

I have a heavily modified OSCommerce 2.2 MS2 website. I would like to have it accept discount coupons that I send to customers. I know there is the "Credit Class & Gift Voucher" mod out there that would do this, but that seems way more complicated than needed for my use (requiring modification of 99 files!). Please note that I have a heavily modified OSCommerce site and you can't just replace many of the files with stock files. I need the following: 1. Ability to setup a discount coupon with a unique code that I can change regularly. 2. Ability for the customer to enter that code to get the discount 3. Ability to apply that percent off of the product total (shipping excluded). 4. That discount coupon needs to work for Paypal, [login to view URL], and check/money order payment modules. Anyone?