Lunar has a couple of nice shopping carts with TONS of features.  The Interchange cart is very powerful.

There are several other simpler shopping cart programs available that plug into most payment gateways.  Take a look at:
http://www.technicallyteaching.com/cgi-bin/cart.cgi
and
http://ttvbfeavo:o1vfevta2t@www.technicallyteaching.com/cgi-bin/store/checkout.pl

(the second one is not being used)

The first one I've been using for months now and isn't bad.

There are also serveral php carts out there.
Just go to google.com and search for "free php shopping cart" WITH the quotes...you'll be amazed.

If you want a copy of either perl cart, let me know.  They're free and publicly available, just gotta dig up the url or I'll just send you a copy.
Hope this helps.