There are a few different customs offices in Berlin. The main Zollamt in Berlin is on Mehringdamm 129c, 10965 Berlin. There is also one in Schöneberg (Kufsteiner Str. 71-79, 10825 Berlin) and Marzahn (Hellersdorfer Weg 35, 12689 Berlin). We’ve been to all of them!
If you are involved with the Zollamt you’ll have paperwork with the address on it. Go there.