It’s not family-friendly, he’s crude, and I feel sort of dirty just for posting this link, but boy does The Oatmeal make some good points here about how to make your shopping cart suck less.
My suggestion: don’t even try to make your own online shopping cart. It’s hard, you know. Instead out-source it all to people whose livelihood depends on getting shopping carts right.
(Funny thing is, before the Internet, people where I come from didn’t even say “shopping cart”. It was supermarket trolley, I guess.)