Not sure where $19.95 came from? The additional driver fee according to National averages about $12 per day. That fee would apply unless you automatically get a free additional driver as per the terms of your rental. Then for drivers 21-24, there would be an additional (per day) underage surcharge on top of that which can vary per location. Renters under 21 may be able to rent under certain circumstances, but in most cases, it's not possible to add them as an additional driver to someone else's contract.