Integrating-Payments-into-WordPress is a project mainly written in PHP, it's free.
Code accompanying PayPal X Developer Network article series on WordPress and PayPal plugin development
The WordPress PHP plugin code in this project can be used to access and display a PayPal account balance in the WordPress administrative interface.
It accompanies a PayPal X Developer Network three part article series on "Integrating Payments into WordPress".
For more information, please refer to the articles bundled together here:
All source code is licensed under the Simplified BSD License: http://www.opensource.org/licenses/bsd-license.php