Does your server support ASP? I am guessing that it might be Windows based and PHP is not installed? If so - you might check out MetaLinks. They have this one that might be able to do the trick.

I have hard of that JavaScript cart a few times in the past but no comments whether it was good or bad came up.

I don't know exactly how it works, but JavaScript being a client side language, that would be something to consider. How are the products being stored (and where)?

Have you also considered PayPal's shopping cart? It might be better and easier.