One question I get a lot of, is how to add a shopping cart to your site.
There are lots of ways, but, one way that I've found multiplies your sales tremendously, is by using a third party marketer such as paydotcom. The reason I recommend them, is that not only do you get an easy-to-add shopping cart to your site...
But they'll hook you up with a bunch of affiliates who'll sell your product FOR YOU as well.
Best of both worlds.
You get your shopping cart, and a bunch of salesmen working for you, endorsing your product :-D