Paypal provides a service called Instant Payment Notification (IPN) which can notify a URL when a payment is made.  Paypal2SalesforceLead is a very simple Product for Plone which sets up a listener for IPN, and adds a new Lead to a Salesforce.com account using a Salesforce web-to-lead form whenever a new payment is made. See https://www.paypal.com/us/cgi-bin/webscr?cmd=p/xcl/rec/ipn-intro-outside for more background on Paypal IPN.