When we book our rental cars, the firms reserve an hour or two of the car's total time for various reasons. For example, if your flight is delayed and you will not be returning to pick up the rental car in time, then you can't just add those hours to your reservation.
Some companies may charge a late fee if you keep the vehicle for longer than allowed. But you will incur additional charges when making amendment to your reservation. However, since this is an important detail, we are happy to contact other airlines on your behalf and make sure any additional costs are covered before returning your car. Make sure you also ask about extending your rental if you're thinking of doing it. There might be a different base fare that applies depending on the time, rental type, and which country you're in. So if you reserve a rental car for five days, the daily rate will be calculated for the original time window. If you change your return time midway through these five days, this may result in an adjusted base price.