Casual Rumblings

I think this is a simple task, but I haven't seen any examples of this on the web (or I haven't looked hard enough).

How do you send custom information with a Paypal button click? Let's assume I want to send an email address or userID or UUID with a payment for tracking. After they do all their processing at paypal's site, I want that tracking variable to be available on the receiving thankyou.cfm page hosted at my site.

Right now it's done via the logged in person's session ID. They have to be logged in, have to have a session scoped ID available and the thank you page will update the DB based on this criteria.

I'd like to change this from a session scoped variable to a coded variable sent from the get go.

Is this possible? Is it as simple as adding a hidden form field ?

Thanks in advance to all you paypal ninjas :)