I don't see why it is any more difficult to return the car empty compared to full. Yes, there are garages that could be en route to the airport, but when we are on the way to the airport, it is easier to be on the way to the airport. By the tim we are returning, I have usually got a good feel for the fuel gauge and can guesstimate how much to add when. Plus, the prices we have been given for fuelling the car have never been unreasonable or excessive, given the variability of gas prices that you can find.