Pay pal had many interfaces.
Ofbiz started with IPN https://www.paypal.com/ipn
Then went to Express check out but changed the IPN code. https://www.paypal.com/us/cgi-bin/webscr?cmd=_additional-payment-overview
finally we have NVP API. https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_NVPAPIOverview
so I am creating an IPN interface that goes back to 777794
I am not familiar enough with express and NVP it they are separate or not.