I have found no visual reference for the post office but these frames taken from the movie and a detail from a photo for the Stern. The access to it was from Jebens Straße or from the station main hall (see previous posts).

Credits: data is credited to “© OpenStreetMap contributors”. The data is available under the Open Database License. | Graphics, adaptation and code are mine and are licensed under the Creative Commons Attribution-ShareAlike 2.0 license (CC BY-SA 2.0).