There is a really simple way to get e-commerce on your site. I have used this method for several sites, and it works very well. It's called Mal's e-commerce:
http://www.mals-e.com/

Mal's site is your back end, and he has his own Thawte certificate. You get an e-mail with an order, but without the credit card number. To retrieve the card number, you log in with two different passwords.

On your site, you just have a "buy it now" button or simple text line, pointing to the shopping cart.