OSCommerce and Paypal Payments Pro Fix

I own a website, [login to view URL] which sales purple martin houses. I have only had the site up for a month or two but have found that I'm having some problems with the ordering system. I have custom built pretty much everything and I"m primarily just using the OSCommerce for the shopping cart system and processing the orders. Everything works fine until they actually try to send the payment. I have contacted PayPal and they say that it is not anything wrong on their end or with the PayPal Payment Pro (US) module. It is something to do with OSCommerce itself. The script as soon as they submit for payment will be sent back to the first shopping cart page (displaying what items they have in their shopping cart). It is not sending any information to PayPal according to PayPal or even attempting to process the order at all. I need someone to fix this problem. It is probably going to be an easy fix and the amount of time will be limited, especially if you are really familiar with OSCommerce.