Let me start by saying that I have only begun working with WordPress for about a month, and my background is in java/c# development. I know barely enough about php to do what I'm doing, and can figure that I am probably not adhering to wordpress coding standards.
I have a PayPal donate button I created at paypal.com, and after the user completes the payment and returns to the site, s2member's auto-return page kicks in. If I recall, it complains about not being able to verify post variables or something (sorry, been a long day. Let me know if I should go back to get the exact error), but I've made an (IMO) ugly but simple workaround that I'm happy with to customize that message. In short, I am directly hacking "s2member/includes/templates/returns/default-return.php", though I am aware this is not the recommended way.
What I am having trouble doing, is figuring out how to send an email with a custom message to the person who just made a donation. Specifically, I just need to figure out how I can get the user's email. I figured I can simply call wp_email in the default-return.php to send out emails as I need to, (which works fine for my purposes) but I just simply cannot figure out how I can get the user's email address from paypal. This needs to work for everyone, (IE, people who are NOT members of the site) so setting up the donation button as a "custom capabilities" purchase is not an option. I'd also like to avoid using the single page/post purchase if I can, but if I have to work with that, then I will.
To summarize my question: Any idea how I can get the user's email address from PayPal on the s2member auto-return page? (default-return.php)