

Unless there was a postal treaty that I missed - hardly impossible, but I'll go with this, anyway - one of these things does not belong with the other.
Having worked the US mail, decades ago, my guess is that this is a simple matter of benign neglect.
Neglect is easy: the clerk applying the cancel, and the clerk sorting the mail, paid attention to the US address, and just didn't notice the stamp.
Benign is easy, too: "oh, who cares, at least they paid somebody ... its not like they're stealing".
(Before you get snarky about the 'benign', think of how many times your letter carrier delivered poorly-addressed mail.)
Folks who've never lived in some of the less-formal countries have a hard time with this sort of thing; those of us who've lived in more than one do not even skip a beat.
Of course, I've been thinking that some tourist put 'the wrong stamp' on a postcard that they mailed home from Puerto Rico.
Perhaps the tourist:
- bought the postcard on Puerto Rico, and
- traveled to the Dominican Republic, and
- bought a stamp in the Dominican Republic, and
- dropped the postcard into the mail at a DR post office, and
- the DR PO never postmarked or canceled the postcard, but
- the DR PO sent all of its outbound mail (or just all of its US-bound mail) thru the US PO in Puerto Rico, and
- that's where this piece got postmarked & canceled.
(More than one small country passed all of their outbound mail en masse to a larger neighbor.)
Q/ Any thoughts?
Q/ Any covers or cards in your collection with stamps from one country that were
only postmarked & canceled in another?
Cheers,
/s/ ikeyPikey