Down a nondescript hallway on the fourth floor [of the 34] of 580 5th Ave., New York, lies an actual post office. There are no signs for it outside the building, and inside there's no sign of it until you physically reach its door on the hall's right-hand side. The only way to know it's there is to view its listing on USPS's online locator tool ahead of time, and ask yourself why there's a location just three blocks south of the huge Rockefeller Center PO.
A photo of the exterior of the building:
The oddest part about the Appraisers Stores post office is that you can't mail a letter there, even if you try. Shipping that book you sold on Amazon? Nope. That care package of cookies to your kid at camp? Fuhgeddaboudit. This post office serves one purpose -- to accept Registered Mail. As it turns out there's lots of it here, since this building is the heart of the Diamond District in Manhattan. Not just at the heart of the Diamond District -- the heart. Just look at the truck out front!
(Actually, the post office can also sell you some stamps -- but that's it.)
For a more detailed history of the Diamond District, a man named Stephen Kilnisan offers tours of this one city block. (An interesting read!) A full view of the building is available here, along with an architectural history of the building itself.
Appraisers Stores was the first post office I visited that requires a security check, including fingerprinting and ID scan, to get to. Although, that wasn't surprising once I realized this is the hub of diamond jeweling in New York City. Heck, that also explains why this post office exists to begin with. Consider: You have a million-dollar shipment to transport. Are you going to take it outside, walk the three blocks, and wait in line at Rockefeller Center's post office three blocks away? ... I didn't think so. You'd probably want to stay within your secure building and ship it upstairs.
I have no idea as to the specifics (and I wouldn't disclose them if I did), but I can only suspect the security precautions taken at this facility are quite high. A former supervisor with whom I started discussing stamp collecting at another post office suggested that many packages shipped from this location require private insurance, well in excess of the $25,000 maximum per package granted by USPS.
In terms of philately, one can't mail out for the Appraisers Stores postmark. Why? Because they can't actually mail it back! Only Registered Mail is accepted, so I believe this one has to be obtained in person (or possibly serviced by a supervisor at a neighboring office). You'd better be ready to prove you're a philatelist; they don't take this postmark lightly.