Basically, all reservations made are charged - regardless of whether you used the reservation or not. The reason is that your reservation blocks the car for other users.
It's best to contact us at [email protected] and we will look into your case in detail.
As long as the reservation time has not yet started, you can simply delete the reservation yourself by clicking on the existing reservation and clicking "Delete."
Create/Delete Reservation
How can I create or delete a reservation?