288983 Changes to existing php script

I have a custom made shopping mall script. [login to view URL] Right now I have it set as they pay a commission based on the price they set their item at, I want to be able to disable this in the admin and just charge a flat monthly fee. Meaning: I can have option of commission based or flat rate w/paypal subscription setup on new user signup. These changes between types can be made in admin area. Right now, each plan (there are 3) have options available to them, plan 1 has basic options, plan 2 had a bit more options and plan 3 have all options available to them (featured items, ad banner, showcase items, etc..) When switching/adding in the new monthly fee, all options should be available to all users who signup to sell items. No limitations. The way I want it is, a user signs up to sell items, they complete the signup form and they are directed to paypal to make their first months payment and a subscription is started, I need it so that if they dont pay they can login but their products arent shown on the website for visitors, understandable? We would need paypal IPN setup, or if possible if they dont complete paypal their signup info is not saved in the system, whichever is easier?? Also, monthly fee should also be able to be set and changed in admin area. Payment made AFTER completion and testing on my server only. Please only bid if you can complete quickly, no excuses.