We used this service and they quoted us one price over the phone and decided to add $10 when we were getting off the bus. The driver said it was nothing to do with him and take it up with whoever took the booking.
We have used this service on numerous occassions and have found them to be very reliable, very reasonable and would highly recommend their service. In fact we no longer drive to the airport I will organise Mark or one of his team to collect/drop off.