We are thinking of renting a car for 1 day this trip - just to do some shopping and see if we might want to rent a car for longer in the future... I went on the Alamo and National rental sites and got prices. The info. says unlimited mileage (I think it comes out to about $35 for the day for a small vehicle). My question is, are there hidden fees on top of this? How do they charge for gas used?