Speeding fines in germany are quite cheap, but they increase fast.
Austria the fines are the same, small infractions small fines but rise fast.
If you dont pay and then revisit and get pulled over you will have to pay the fines and charge for late payment, they can also sieze your vehicle and sell it. So germany, austria and switzerland pay the fines and be happy.
But contact the police that sent the letter, there are scammers rings sending these kind of papers also. So verify that it in fact is a real ticket you got.