Paypal Payment Gateway Integration

https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/apicertificates

https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_NVPAPIOverview#id09C2ED00ZY4

https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_Appx_websitestandard_htmlvariables#id08A6HH00W2J