Hahaha, yeah, it's 70 cents, don't worry. When I was in Berlin I even used a 55 cent stamp once and my parents got my letter anyway. ;) Even if a few times the envelope arrived without the stamp because someone had stolen it. D:

Ah, the post office at the Neukölln Arcaden, I had to go there once too. Come on, that's not too far away. But maybe that's just me being used to a very bad service, so these problems seem very small to me. ;)