How our software works is how it was explained. Your free since the software is not encrypted to modify it however you wish. If as an admin you elect to charge $5.00 for an ad then the software will correctly charge $5.00 times the quantity of ads the user elects to sell. It is not $5.00 for unlimited ads as most customers in the classifieds business would consider your thinking a bug as I stated before.
Most people since the charge is per category create categories to house different types of things. That way you have a range of categories for different things.