How would you get this done?

[eluser]jedd[/eluser]
[url="http://ellislab.com/codeigniter/user-guide/libraries/sessions.html"]Session data[/url] is your friend.

Write a function that tests if a piece of user data exists in your session - if it does not, create it using some method of your choosing - the md5 of the session id, a timestamp, a random string, the actual session id, etc. Obviously the function will not overwrite this piece of userdata if it already exists.

All subsequent functions involved in cart adds, tracking, emailing and so on, will refer to this piece of data.