Hi there,
I have been programming in php for about 10 years and have chose shopping carts as my specialty. I decided years ago that I would work with oscommerce as the free open source cart to code for.
I have done wonders with oscommerce over the years, from contributions, contributions on top of contributions to a cart that has over 100 contributions. I also have published on oscommerce in the contibutions section a custom payment module for a company called propay (to this day I am the only one to do this).
I have installed this module enough times that I can install this in less than a day for you without affecting any contributions that you may have installed on your site.
Please see my reviews for the work that I have done on freelancer.com.
Thanks for your time and consideration,
Shawn Mulligan