PayPal Button Question - Need some advice!

I've recently created a web-based program that notified people of certain posts that match their keywords on multiple forums. The program has been doing real well and I'm at the point now where I need to start thinking of making it a subscription-based service. With this in mind... What's the best way to handle their activation with a PayPal button?

So far I can create a "subscribe" button with a monthly and yearly option on it. The area where I'm struggling is the checkout URL. If I just make this go to a "Thank You" page I'll have to add some text that says "Please allow up to 24hours for your account activation" so that I have time to get into my email, figure out how subscribed, note their email address, get into my program, and activate them.

However...

If there's a way that I could pass their e-mail address along with the checkout URL, then I can just target a CGI script that auto-activates them and then says "Thank You". But the problem with that is two-fold, 1) I don't know how to pass the e-mail address and 2) Other users could just target the checkout completion script and put in their own e-mail address and become subscribed without paying. Granted, that's pretty unlikely, but possible.

Regarding passing the e-mail address in the URL, PayPal as the option for adding "Advanced Variables" but they appear to just be static variables and not any that I can use to resolve and then pass.

Please let me know what your advice is!