Hi!
I use PhotoCart from PicturesPro.com
http://www.picturespro.com/pages/pho...shopping_cart/
I don't use it to its fullest potential, but it has made my life so much easier. And my customers find it a snap to use.
I know there are other similar services, but this one is fairly resonably priced and you only pay once and you're done. No montly fees or fees based on sales or anything.
Good luck!