Visiting the Post Office is something many people here loathe. It's a prime example of German bureaucracy. It's possible to buy stamps at a machine, as the woman on the far right is doing, however you have to know the exact envelope size, weight, and destination are three variables that will determine the price. If you need change prepare to receive it back in the form of stamps.
Any uncertainty requires standing in the endless line. Usually the postal worker pulls out their handy drawer to measure exactly. While you're at it, forget getting your mail delivered if it has anything than the name of the resident that is registered at the KVR. Not even addressing something to 'Oma (insert last name)' will work.