I suppose it's a polygon, but my PO Box has its own ZIP+4.

APO/FPO is a good counterexample, though even these presumably have domestic delivery points where the postal service hands off mail to the military (not that their locations should matter to postal customers).

Same goes for DPO addresses, I assume USPS doesn't actually deliver to embassies overseas (or at least does so under contract with the State Department if it does, and again consolidates deliveries at one or more domestic locations).