PayPal NVP API

The PayPal integration XenForo uses is the old school "Website Payments" system that PayPal supports via creating a form and submitting it to PayPal. It works, but PayPal has an exponentially better API now that let you do a LOT more than just blindly take payments.

https://cms.paypal.com/us/cgi-bin/?...t_ID=developer/e_howto_api_nvp_NVPAPIOverview...

Read more

source https://xfworld.net/threads/paypal-nvp-api.47167/