These things are tremendously difficult. That's not to say "so hard that you can't do them", because you may well be able to do them in practice after a considerable amount of work and you may very well be better at this sort of thing than me. :-) Nevertheless, they are difficult to implement and the standard solutions that exist tend to be pretty expensive, aimed at reasonably big businesses.

You might consider being an amazon zshop or using Yahoo!'s similar shopping facilities as a reasonably accessible way to solve the problem, but at a very considerable cost to flexibility (as well as likely a siginificant financial cost too).

Good luck!