If they don't they likely will soon enough? They sell ammo, they should know laws governing ammo. Most, if not all online sellers post ammo shipping info and how it's shipped. From what have read, it needs to be signed for as well.I looked at the USPS website, and it says ammunition is prohibited from shipment, but like you, I think I have received ammunition in the mail before. I just had a FedEx delivery today of some 7.62 x 51, and it had a hazmat code on the box. Maybe some shippers don't know the federal regulations?
On a side note, around my place, carriers including USPS, routinely deliver for each other at times.
So, USPS may well have delivered for FedEX, UPS, DHL or others?