Shopping Cart Software
There are several different solutions that you should consider for your shopping cart software that we will go over here. However, you need a lot of flexibility, you should probably consider another solution. My personal favorite is osCommerce. This is an open source (so its free!) shopping cart solution with a big, very active community to help you out along the way. Your host will need to have PHP and MySQL for osCommerce to function. It is very simple to run an unmodified version of osCommerce. You just download it, upload it to your server, and run the web-based install, or if you have Fantastico, you can automatically install it from there.
This is a necessity of any ecommerce business, because you need to be able to take payment in your store. There are several different ways you can go here. You can either go with a 3rd party processor, or a true merchant account which is built into your shopping cart.