I misinterpreted the "standard fee" phrasing in the airport's website (and yeah, given that it's a german airport website, I realize it's probably not written by native english speakers).
Even with my very rudimentary command of English I would not understand a "standard fare" as a flat or lump sum fare but as the fare you have to pay under normal circumstances....
There is no city in Germany that allows flat fares or lump sum fares within the city borders. All fares within a city border are metered. As soon as you cross the city border you are free to negotiate. This ends up in strange results: I pay 14 Euro metered fare to my (local) railway station (5 km from my house) whereas I pay 10 Euro flat negotiated fare to the long distance railway station in the next town 8 km away.