In Spain, Car Rental companies have a fixed stock of cars for rental. If there is a demand these companies can rent more cars from a kind of Car Rental Wholesaler. Most Spanish airports have these. So if you pick up a car at one airport there is usually no problem with returning it to another airport. Yes, there is a fee (usually circa €35.00). When you are booking the car, the facility will probably not be available online and you will have to ring the car-broker (not the wholesaler). Believe me Spanish Car Rental outlets and brokers want your business so don't be afraid to bargain.