I got a great rate on an Alamo rental in Hawaii back when cars were available and prices were reasonable. I need to extend the rental by one day. Modifying the reservation is not an option. Can’t start a new reservation because no cars are available. That said, when is the best time to extend a rental, when I pick up the car at the airport or after I drive off? I tried calling the toll free number and Costco but they are unable to change the reservation without the rate doubling. Thanks for any advice.