PHP mySQL store script - addon

I currently have a shop using Shop Script Premium. It works great but I would like to add a coupon feature and the original writers can't/won't do it. It has fully open source code so it can be edited however I want. The shop runs on PHP and mySQL and I have customized it as far as I can with some PHP and HTML editing but now I need help. Do you think you can help me out? I would like to add the coupon feature so that I can advertise and customers can just type in a code that I present and then advertise. ## Deliverables I would like you to edit the scripts for me or provide me with exact instructions on how to edit it myself. ## Platform Windows/Apache Server