I happened to be in New York City on 9 February 2018 so it was an opportunity to investigate. To enter the United Nations, I had to check in across First Avenue
https://www.google.com/maps/@40.750...13312!8i6656 see where it says, visitor check-in. And yes, I had to show my ID which they scanned.
Then, with a label displaying my name, on my jacket, I went to the visitor entrance.
https://www.google.com/maps/@40.751...13312!8i6656 to go through a security checkpoint about the same as an airport or courthouse. The post office is downstairs, and they had one cashier on duty selling postcards, and stamps, and carefully asking whether the item is domestic or international to avoid wrong postage. There are two mailboxes the same as USPS street collection boxes but light blue in colour, same as the UN flag, and into these go anything mailed with United Nations stamps. I did speak to the supervisor about how they really should make it easier for UNHCR's and Unicef's United States affiliates to send mail through UNPA in order to use up all those older stamps available far below face value, which would help those agencies save postage when mailing out letters asking for donations. I also shared this suggestion today on line with Unicef USA.
For a short time there were two cashiers on duty; and they have three cash registers altogether.
*** Moved by Staff to a more appropriate forum. ***