Add ''buy 3, get 1 free'' deal to OSCommerce installation

I have an OSCommerce store, that needs to have a "buy 3, get 1 free" deal added to it. All of the products offered are the same price, so it shouldn't be difficult from that angle. I'd like it to be selectable, and the number you buy to be assignable in the admin interface. I envision a checkbox to toggle whether the offer is available, and a small text box next to it that you put in a number (in this case 3) of products that need to be purchased to get one item in the cart for free. The coder will be given ftp access to the site, I'll be able to load any .sql file changes into the database. ## Deliverables 1) Complete and fully-functional working program(s) in executable form as well as complete source code of all work done. 2) Deliverables must be in ready-to-run condition, as follows (depending on the nature of the deliverables): a) For web sites or other server-side deliverables intended to only ever exist in one place in the Buyer's environment--Deliverables must be installed by the Seller in ready-to-run condition in the Buyer's environment. b) For all others including desktop software or software the buyer intends to distribute: A software installation package that will install the software in ready-to-run condition on the platform(s) specified in this bid request. 3) All deliverables will be considered "work made for hire" under U.S. Copyright law. Buyer will receive exclusive and complete copyrights to all work purchased. (No GPL, GNU, 3rd party components, etc. unless all copyright ramifications are explained AND AGREED TO by the buyer on the site per the coder's Seller Legal Agreement). ## Platform php, mysql