I'm trying to set up account upgrades and I'm a bit confused on how this works. I have a business paypal account and have enabled Instant Payment Notifications as the help page specified. The problem is the part where it states: "You should enter a notification URL of [your Board URL]/payment_callback.php". I've added that for one of my forums, however I currently have 3 xenforo forums and would like account upgrades to be available on each. How would I set this up through paypal when I only have the option to add one URL? I guess I'm just confused on what that URL actually does.