Thanks, I was unaware of street_cabinet=postal_service, which seems to be more common than amenity=relay_box. It seems like both tags came up in the October 2014 tagging list discussion about street cabinets. However, I’m unsure about the relative advantages of either approach. It also isn’t clear to me how to distinguish between postal relay boxes and other postal storage boxes. That would be a minor point except in the kind of detailed 3D renders we were discussing previously.
https://commons.wikimedia.org/wiki/File:Storage_%26_Relay_boxes_(7855000056).jpg