Hello we (staff of 20) are a well-known web development team specializing in designing and maintaining major ecommerce sites especially sites of 1000's of items for the local textile and jewelry businesses in Surat. Actually, the company's origins are from setting up secure merchant payment gateway options online. Combined we have up to 26 years of experience, expert skill, and industry leading know-how to configure and secure your payment gateway properly. Not everyone can be confident enough to make that claim. We guarantee your needs are met with accuracy, on a timely manner and with a fair price.
I have looked over your code and your site. I see where the shopping cart does work and displays the total amount of the items but does not list the items that the customer is purchasing. I understand items list with the quantity must go on "CDSC=Descripcion" replacing the word Descripcion. Ultimately, you would like to have the item and quantity of each item to be displayed when checking out.
(Also the red "checkout" link seems to have issues on the top right side of the pages. Also the "back" button took me to an error page and I had to "refresh" multiple times.) Not to worry, for the Shopping cart issue, I am confident we can solve the issue easily since we setup and maintain Shopping carts for PayPal all day for about 365 days a year with sites much bigger than your site.
You see, we are specialized in ecommerce, PHP, .NET, and etc. With our team of 20 you can be certain we can get this done as fast but accurate as possible.