CCBill Postback System

I am looking for a low cost solution for the following : Basic Open Source or custom Shopping Cart System (very Basic) 1 page with products (downloads) listed and facility to add items to cart and checkout via CCBill postback. Currency of the system is to be tokens. User purchases 1, 5, 10 and 15 tokens (I will setup CCBill system before developer commences work) via script with CCBill as payment processor, CCBill system posts back info to the script upon successful payment and users account is credited with the relevany token qty. User purchases download items from store with tokens until tokens are used. Once credit is used, user is prompted to purchase more tokens. User to have history on all credits and purchases via a 'My Account' button. Any user is to be able to sign up for free, then can purchase tokens via CCBill postback for downloads. Admin to have facility to add products, assign credit price to product, product thumbnail and a complete purchase and credit history by customer username. The developer must have knowledge of the CCBill postback system.