Many many years ago, as I was getting into the hobby seriously, I had some 9mm antique German largo WWII box unopened ammo that I sold to a collector in Maine. I checked the USPO regulations and failed to see any restrictions on shipping ammunition via the mail. I boxed it up nicely, took it to the UPSO window in our town and shipped it priority mail to Maine. I even told the clerk what was in it. She didn't even wrinkle her unibrow. Two days later it was there even though the consignee was throwing fits that I had shipped via mail. Later on I discovered it was a no-no but no one at our station knew the regs.
Since then I always use UPS with an ORM-D label. It's so easy that there's no reason not to put the tag on. It just precludes that box from flying, that's all.
Rome