Hi.
The post office is already mapped in https://www.openstreetmap.org/node/1083758245
The tagging scheme used is this one: https://wiki.openstreetmap.org/wiki/Proposal:Shop_as_post-partner
In case organic maps does not show this correctly, you could approach the developers of that app. I did the same for OsmAnd, see https://github.com/osmandapp/OsmAnd/issues/16186
Hope this helps.
Matthias