I can't find any documentation on the pay per post plugin.
When I go to Settings it has a yes/no – but I have people creating several different kinds of posts. I only want to charge for one kind, and advertising post. Also, can I set the post to expire if they don't renew?
Here is some documentation. If you select yes, then it'll be activated on all posting forms. Their posts will get pending and will be published when they are paid. You could expire the post, that needs some coding customization. There are filters and actions available on the plugin. Filters are not yet documented.