A nice tool for adding items to a shopping cart has been created by a company in Portland called Panic. The idea is interesting and makes a lot of sense. You can click on an item you want to buy and drag it into the cart window. It inserts the item and calculates the total.
I think it is a great step and something to learn from. I would be interested to know how useful it actually is to users.
Have a look: http://www.panic.com/goods/