Need Help - Adding information to a database after purchasing something with paypal

Ok, so I am selling software online. My program uses an online database to check if the user is allowed to use the program, and so far I have had to manually add the users to the database for the info to work.

I want users to be added to the database on successful purchase of my program through the buy it now button on my website. The only way I can think to do that would be through a php page that a user is redirected to after purchase (a feature of paypal), but I don't see how that would be secure as any user could just type that in their browser.

I also think that I could add code to the button to submit a php page, but then if they do not purchase, they are still added.

I also need to note that the paypal "buy it now" button has two options that a user must input for it to work - a desired username and another field.

Ideas?

Paypal has a thing where they return after a "successful" transaction,
along with an IPN (special key). Read more about that on their development site:
https://developer.paypal.com/

In fact, you create a sandbox account and use it to test your site exactly like
your REAL paypal site, using fake credit cards. Once you get it working, you switch
back to your real paypal account. A ton of great developer tools too.

EDIT:
Keep an eye on this thread ... similar to what you are doing?
http://www.codingforums.com/showthread.php?t=190399


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