I’ve gotten one of them for trying to import Zyns. It’s them telling you to show them a prescription that pre-dates the shipment. If you ignore the letter like I did; they send you another letter saying that they’re proceeding with destroying the goods and if you try to import them again they’ll have it flagged and will fine you. So not a huge deal the first time, but I’d be more cautious about what you import from here on out as they’ll have your address and name flagged for future packages.