Software, no. Possibly helpful scripts, yes. I've been using jquery on a lot lately. There are helpful tutorials out everywhere. I haven't used a shopping cart one yet, and I don't really know if this is what you're looking for, but I figured it couldn't hurt.
http://www.redfoxmagazine.com/10-jquery ... rt-plugin/
The first example on the page is for a Paypal cart. Don't know if that's what you're using, but I know its a bit of a standard. If you click the image and go down to the example it gives a working example, the code, and spells everything out for you. Plus, it tells you how you can customize it. Win Win.