The concept of "fixed rate" to/from the airport is a USA-ism I haven't encountered elsewhere in the world, where the meter rate is always standard.
By the way, regarding tipping 0.50 on a 21.50 fare, if you did that with a taxi driver in a number of US cities you would get a punch on the nose.
I always enjoy the taxi ride out to Dusseldorf airport from the city, it is not too far and although there is only about 3 miles on the Autobahn the drivers always get their old Mercedes up to about 100mph if they can !