You have no legal obligation if someone mails you a live cartridge. They have commited a Federal Felony. You might want to report this to the PO but this is not a legal obligation. If you mail it to someone else THAT is a problem.
Paying big dollars to send a cartridge sucks,no doubt. Defending against a Federal felony charge is a sucking hurricane.
A big problem with this is that the PO is considering prohibiting the mailing of INERT ordnance . Some European countries have already done this. That really puts a damper on collecting. The business between the US and Europe is down 99% because most sellers will not ship to the US any longer.
Their argument is ; “how do we know that the item is INERT”? That is a valid consideration. The fact that folks are sending LIVE Civil War shells through the mail supports their argument.
I expect that the PO will prohibit mailing INERT shells and other ordnance items before the current administration leaves office. That will mean bullets , cases , dummies etc.