forgotz writes:  

DaDaBooks 2.5 is a full featured PHP-Nuke PayPal Accounting and Invoicing system, allowing for back-end administration and invoicing, as well as front-end client login to access billing status.

Download and screen shots (http://dadanuke.org). Registration is required for download.With DaDaBooks, PayPal accounting and billing is a snap! All from the comfort of your PHP-Nuke site.