Originally Posted by
SwissCircle
You would need a residential adress in germany. There is no "free roam" of people in this country. Every citizen needs to be registred, so anyone can check where you actually are or supposed to be. Heavy fine for non-compliance.
Alles Kontrolle here...

At least far more relaxed than in Switzerland.
It is only a problem with Deutsche Post AG as the German member of the UPU According to their TOS you need to have a valid adress in Germany to obtain a mailbox with them (and this has nothing to do with Germany but with regulations of the UPU).
There are several private companies that offer street adresses or mailbox adresses in Germany.