title: Paypal - warn website on payment
body: <p>I'm building a SAAS based on a credits system, which are bought via Paypal.</p>
  
  <p>Right now, I use the Payment Standard with PDT to check that the payment has been done correctly. When the transaction is done, Paypal redirects the client to a page on my website which checks that the transaction went well, and gives the amount of credits matching the price paid.</p>
  
  <p>This works fine, but I have a problem : since the website need to be redirected from Paypal to add the credits, if the client is not redirected for whatever reason, the customer pays but receives no credit.</p>
  
  <p>My question is thus : Is there any way to warn my website from Paypal in a more robust way, and is it possible to cancel a transaction in case of error, and How ?</p>
  
  <p>Thanks in advance</p>