Ok, here's my take on this.
39 USC 410 (b) The following provisions shall apply to the Postal Service: (2) all provisions of title 18 dealing with the Postal Service, the mails, and officers or employees of the Government of the United States.
This in my opinion does not mean that 18 USC 930 doesn't apply. Because a post office is a federal office on federal property, right? 18 USC 930 (a) Except as provided in subsection (d), whoever knowingly possesses or causes to be present a firearm or other dangerous weapon in a Federal facility (other than a Federal court facility), or attempts to do so, shall be fined under this title or imprisoned not more than 1 year, or both. (d) Subsection (a) shall not apply to— (3) the lawful carrying of firearms or other dangerous weapons in a Federal facility incident to hunting or other lawful purposes.
Nowhere in 18 USC 930 does it say that the post office is excluded, just federal courts.
So, is the post office a federal facility? Yes, it falls under federal jurisdiction. There are other areas of 18 USC that don't talk about federal facilities or the post office, they don't apply. But I believe 18 USC 930 does, because the post office is a federal facility. Otherwise it's a private facility or a state facility and then isn't covered by any federal law, including 39 CFR, which is a federal regulation.