As you cannot really include something like that on the PayPal Checkout Screens for Payments Standard, one easy option is to have your policy clearly stated on your web page or site. Another option is using Auto Return whereas the customer is returned to a page on your site after completing the checkout - most people use Auto Return for displaying a Thank You Page however, you could also have your policy displayed there. You could also use PayPal's IPN or Instant Payment Notification feature whereas you could generate a customized email which could be sent to the customer. And there's 3rd party carts to consider - many of these carts will show what was purchased before sending the data to PayPal for checkout. Last option would be using Payments Pro whereas you control the checkout experience - the customer remains on your site while behind the scenes, the credit card info is passed to PayPal for processing. Note, Payments Pro has a $30 monthly fee plus the standard transaction processing feed.
