Oscommerce module install

need help for downloads and installation free payment module for osCommerce. I do not have the "add on" or "plug in". Coder would have to have, find, or create the code to work. Object is to have customers (after creating and logging into account) to select a downloadable product. If the downloadable product is "Free" then the user would not be charged for file, would complete sale, and download link is present for customer. IF download is a retail item for say $2, then the item would go through the process and be charged $2, then download link provided. #1 make downloadable files (products) work. #2 install free payment option for Oscommerce to work.