Instant Payment Notification (ipn) in your account profile can be activated in order to receive a notification of payment or other info.
You need some code in order to receive, interpret and then perform the desired action from the data received via the ipn.
I think I remember Paypal furnishing php code to do similar to what you want, with a little modification.
Sometimes you have to dig deep to find info on their site. Probably in the integration center.