That'll piss customers off big time. There's plenty of reasons why a customer might legit cancel - maybe the husband books before the wife is ready or the baby kicks off just before the taxi turns up. A fiver was the right number I'd suggest?
Or how about this...
Got a street hail this morning to the airport. They'd originally pre-booked a FN taxi but it looked like the driver was trying to do another airport tun before picking them up. He was 20 mins late and was refusing to cancel their job. Cheeky fukker looking for the fiver I guess? So they had to cancel the job and immediately got a €5 cancellation fine. I told them to write to FN explaining their situation but they probably won't.