How do you identify users when using PayPal?

Question

From this article on CodeGuru about PayPal IPN, I see there are two fields, payer_email and payer_id, that can be used to identify the user. Is there anything else I'm missing? Do you use these fields to identify the user or how do you do it?

Thanks

Answer 1

I've used the 'custom' variable to hold the user id in my system. Then, when the ipn notification comes back to my site, I use the value in 'custom' to look up and identify the user when processing the ipn.

Doing this requires the user to be logged in on my site so I know who they are and can add that information into the 'custom' variable for the paypal button.

Answer 2

Can I use this 'custom' variable even in subscriptions? My question is if this 'custom' variable will be passed to me in the next billing cycle.