Hi,

In your PayPal account click Profile, then look in the right column there is a link for Instant Payment Notification.

There you put the url http://yoursiteroot/Services/PayPalIPNHandler.aspx

and you enable IPN in your PayPal account.

Hope it helps,

Joe

Its best to have both enabled, there is no problem. The PDT happens when the user is redirected back to your site and the IPN happens independent of whether the user is redirected back to your site. They don't conflict, they are just redundant in the case of both notifications occurring.

Best,

Joe 

Using the PayPal sandbox is very difficult, I have no magic solution. Its not really possible to setup a public demo using PayPal sand box because it requires the developer to be logged into the sandbox to work. I only test with the sandbox on localhost then on a public facing site I use production PayPal.

PDTId is not the same as merchant, in fact for PayPal Standard there is no setting for merchant id there is APIUsername, APIPassword, APISignature, Email, and PDTId, they are all different values.

Hope it helps,

Joe