In order to enforce your post limitation until the user pays the license, I suppose you should check if limit is reached when:
- user clicks on “add new”
- user displays list of current entries
AFAIK, the way to do that is to use the relevant hooks but that depends a bit of the user experience you want to create.
For example, if you use the action hook publish_post
, you could force the “draft” status of the post until the user has paid.
See the full action hooks list here.