How can I ensure that the PayPal payment is returned to my website to credit the user immediately?

Well, I'm working on a site that basically accepts PayPal payments to give my accounts premium status, no need to go into details on what it does, but what I'm trying to accomplish is "pay now" with with PayPal "which will accept the payment and then send them back to my site with some sort of verification code, so I can say," Okay, pay, let me run this code to set your account to premium. "

Any idea how I can do this? I can provide a success URL, but it requires the user to hit the "Return to *" button after they have paid, and even that gives them a dire warning.