That is easy
The permissions work just like category permissions.
You must set the global usergroup permission to allow payment to yes for whatever usergroup you want to collect payment from. If thats set to NO you are never going to have to pay.
You then of course set up payment settings example to use paypal and enter your paypal email in admin
You then set up payment amounts on categories you wish to collect payment for. If a category has no payment amount it is free to post ads there.