Hey,
Off the top of my head, I would approach that project from a fairly simple point of view.
One way of doing it is creating some sort of login credentials but ONLY on successful completion of the PayPal payment. So in other words, once you have confirmed the payment, you could generate a login for that user, and store it in a local database. Then all that is required is a simple login script to view any of the files within that protected folder. I believe it would be a simple way of controlling entry.
Only those that have completed the small fee would have a login created.
As far as a tutorial I am unsure, but I think that would be a reasonable approach. The only thing that may need some research is how PayPal interacts with your site. How does PayPal confirm the payment (I have very limited experience working with PayPal)? Once you know that you can write your code based off that confirmation.
Hope it helps,
php_dev