In postnuke and php-nuke is a paid module.
But for any programer:
PayPal developer website
PayPal IPN free class
http://www.micahcarrick.com/v2/content/view/1/3/
PayPal IPN Tools:
http://www.paypal.com/cgi-bin/webscr?cmd=p/pdn/3p-solutions-ipntools-outside
Is a good idea many user make a donation(US$10,00 ?) for paid a developer to release a GPL module.