Is there a developers guide or documentation for V 4.0 that really explains about ecommerce & paypal interation? Where the pages are - how to customize it?

I am new to Kentico. I'm trying to help this non profit with their website.

They had a pay membership that allowed for the user access to their forums and newsletters. The pay part was disabled. (Not by me) When it was disabled now everyone who signs up for the forums can get to all the newsletters. They just want a payment on the newsletters.

They have a store where the payments through paypal work.

The non-profit says they never had to enable anyone by hand for the forums or the newsletters before.

So is there some documentation that will explain - when paypal comes in with a "this got paid for" message - where are the pages that pick it up and what do they do with the message? The users name and info does get posted to the database - and somehow they do get "enabled" -

From a previous post it said make a new role - then with Paypal comes in and says it's paid for, add that person to the role. Well I'm not sure where that's at.

And again they have V 4.0

JS