PHP-KeyCodes can automatically send out pin or license key codes after payment from PayPal.
It now has a feature that allows you to send a code to a customer without them making a payment.
The customer enters their email address and the next key code is sent.
You then have a list of customers in your database which you can extract and use for marketing purposes.
For a full description of PHP-KeyCodes refer to http://www.withinweb.com/phpkeycodes