I believe that priceline clearly states that you may be charged extra for underage.
ok i looked it up quote from priceline.com
"Minimum Age Requirements
Priceline.com offers car rentals to drivers between the ages of 21 and 24 years old in select locations. Rentals for drivers between the ages of 18 and 20 are available in New York only. Enter the details of your trip and we'll tell you if we can fulfill your request.
Those companies that allow renters under the age of 25 charge Young Renter Fees. These fees typically start at $20 per day and can be as much as $50 per day for high cost locations or more desirable car types. For example, a rental car that normally costs $30 per day could cost an underage driver at least $50 per day.
With priceline.com, you should include these additional fees in your offer price. If your request is accepted, you will not be charged any additional Young Renter Fees at the rental counter.Please note: All drivers will be required to present a valid driver's license in order to pick up a vehicle and each driver's age will be verified at the rental counter. If the driver's age was misrepresented during the offer process, the driver will be subject to underage fees at the rental counter.
If we are not able to fulfill your request and you still need a rental, click here to see other options from our partners who rent to drivers under the age of 25. To prevent surprises, be sure to inquire about the Young Renter Fees that will be charged to you at the rental counter."