I thought thats how paypal worked also, have a look into paypal IPN.
https://www.paypal.com/us/cgi-bin/we...-intro-outside

When your customer purchases from you via paypal, as soon as the transfer is complete paypal will post back 'behind the scenes' data of the transaction to your site so you can then create a system to deliver the product or whatever.

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mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)