I wrote up approximately everything I know about shopping carts, plus everything I learned from my recent experience rewriting one, and made a proper article about it. It ginormous. It is also probably one of the best things I have ever written regarding my uISV, so if the topic interests you, go have a gander.
It includes a bunch of high level theorizing, some stats from BCC, and more code than you can shake a stick at. (I released the whole cart into the public domain.)
Why put it on my site rather than the blog? Well, it uses live data and integrates directly into the site, for one thing. For another, I’m kind of hoping folks will say “Ooh, that is some wonderful stuff” and link to it.