Hmmm... there is the option of having a link count... it would work, but it would also possibly cause trouble for the shopper. What you do is have a count, so that the page with count 3 has all its links contain a count 4, and if count 3 is seen again (ie someone copied and mailed link) then drop that session. This would work, but has definate shortcomings. 1) no back button for shoppers, going back will screw them up. 2) they can't have two windows open looking at the same session info. Other than that, it would work... tho cookies or IP (with IP not even being great because of firewalls and proxies) would be best.
                - Ant