Using a different PayPal IPN URL per transaction

Question

I have seen and know how to use IPN for paypal payments but I am having a small issue. With IPN you need to enter an IPN address to your PayPal File so it knows you got sent the payment (Instant Activation of Product). If I have multiple sites, I can't do this as It only allows one.

I have seen the WHMCS system manage to do this using Website Payments Standard (Does not require any IPN Settings) where the user's invoice is marked as paid after the payment is sent. The user does not need to be redirected to another page and as a seller, the only info you need to put in is your paypal email for things to start working.

Does anyone know how this works and where to learn this?

Thank you for your help

Answer 1

You can specify a per-transaction IPN URL by passing in notify_url along with the other data in the form.
E.g. <input type="hidden" name="notify_url" value="http://....">

This will override whatever you have set up within your PayPal Profile.