Yeah, HerrGlock's got ya!
LEGAL carry inside a post office is not a, 'huge no-no'. The prohibition against post office carry is not a federal law. It is a federal regulation which, in strict point-of-fact, cannot be federally enforced inside any post office building. Why? Because the post office is not a federal organization; and post office buildings are not owned by the federal government. The United States Post Office is a privately-held corporation. All you're doing when you LEGALLY carry inside a post office is disobeying the property owner's wishes.
Disallowing (otherwise) legally carried firearms in post offices is an example of what is called, 'national fiat legislation'. This rule is a residual government regulation, passed without valid congressional consent, and left over from the antithetical 1990's Clinton administration.
(Besides, as every well-informed American citizen knows: It's the guys ON THE OTHER SIDE OF THE POST OFFICE COUNTER that you really have to worry about!)