Proposed features/amenity=post point
In Belgium, there are "post punten", literally translated as post points. These are shops (like supermarkets) which work together with "Bpost", the Belgian mail agency. These shops have a corner where they sell post stamps. These aren't full post offices: you can't send a special letter from it. So I think this could use a separate tag.
When the shop is a node, add a tag "amenity=post_point" to it, when the shop is a building, put a node in the building (where the post point is) and tag it with "amenity=post_point". Other tags like "operator" can be added.
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