Billing Scrpt Modification and Enhancement

We have a current billing script that uses php and mysql. This script has been in use for over 3 years and works perfectly, however we are wanting to add some additonal enhancements. Currently the script allows users to manually process credit cards. We would like to add the feature of allowing users the ability to create online products and past a button code on a website. This was online customers can order online without having the user manually process their order.