As we know that fo integrating paypal we needed to pass the value in hidden field . which have  a fixed name. please check the link for the list of hidden field name

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

C

heck the link For how to integrate the paypal

http://www.codeproject.com/Articles/19184/Use-of-the-PayPal-payment-system-in-ASP-NET

http://www.codeproject.com/Articles/42894/Introduction-to-PayPal-for-C-ASP-NET-developers