Subscriptions and paid content
To offer paid content on your site, use any subscription-extension to assign the user to a different Joomla usergroup when payment is recieved.
See subscription-extensions in the Joomla extension directory.
We highly recommend Akeeba Subscriptions because it is easyest to configure, works well and is very much compatible with all kinds of other Joomla 3rd party extensions. Besides that, it is totally free.
Unfortunately Akeeba Subscriptions has stopped. We can currently not recoment any particular subscribtion-system. If you find something which works well for you, please let us know.