Before working to chase down National on this I would make a very close comparison of the rates & fees as reserved vs. as charged. Rates for a vehicle class are guaranteed (and the OP should not have been charged more for a higher class unless he requested it); state/local/airport taxes and fees are NOT guaranteed, nor is the exchange rate. These elements can plausibly add up to the difference reported if the car reservation was made some months ago. If a base rate change is confirmed, this is a great use of a Skype call to U.S. customer service.