I stumbled across http://www.e-junkie.com which looked like a good solution for my [simple] ecommerce needs. But it uses thickbox with the latest jQuery, and broke when used in Drupal.
However with the jQuery Update module, it works great.
Long story short, you can use http://www.e-junkie.com shopping cart with Drupal 5.x by installing the jQuery Update module.
Regarding E-Junkie: I found the Ubercart module needs a little more development, and ecommerce module is overkill for selling just a few products. Most proprietary shoppings carts are expensive and create all the product pages and storefront stuff. I didn't need that. All I wanted was to create the product pages within Drupal, paste in some 'add to cart' buttons, and have the shopping cart be external to Drupal. E-junkie satisfies this. It handles product options, coupons, taxes, shipping, etc, then creates the button html. It uses jQuery and thickbox to produce a nice looking cart. It works nicely with PayPal Standard and Pro, among others. Finally it's cheap, in fact free for non-profits.