Answers revealed below
“A guy I know has been putting An Post to the test by mailing random objects and making them solve puzzles to get the correct address. Amazingly, they seem to get it right almost every time.”
Dice. Delivered two days later.
Mobius strip. Not delivered.
Clear plastic with address. Not delivered.
Crossword puzzle address. Delivered.
Letter with jigsaw address. Delivered with ‘a polite note explaining why they opened it’.
Address using pop-up mechanism. Delivered the next morning.
More at link below.
G’wan the An Post, in fairness.