How rental cars work is that we rely on the returns to flip them and re-rent them. A reservation is made, NOT to reserve the physically hold the car there, but for the agency to make sure that the specific sized vehicle reserved is present. Never will a rental agency hold a mid-size car for example if you're reservation is set up to pick it up in 3 days. Being "sold out" online simply means the date you are trying to rent the vehicle is too close to the time you want to pick it up; in the best bet, don't give up, but call the rental agency directly. At an HLE location, we don't guarantee a specific car, but a size of a car. Hertz does not want to have a car sitting for more than 4-5 days at a time, this is considered a NO-REV, meaning the car is sitting there, developing no revenue. The name of the game is flip them as fast as you can, make as many reservations as possible to account for no shows (which occurs 1 too many times). This is why rental agencies become overbooked, because we know we have to account for no shows.