Hello,
I think the street the building No 24 is located in is not Wallbrunnstraße, This is rather Schützenwaldweg – according to the Stadtplan Lörrach (https://www.loerrach.de/stadtplan). Wallbrunnstraße 24 (the location of the new Post office) is close to the intersection of Wallbrunnstraße and Brühlstraße. I hope others will find this suggestion useful – by not looking in vain for a post office at this position.