Here are the regulations of the US Post Office now prohibits mailing:
“Money (coins, cash, currency, paper money and negotiable instruments equivalent to cash such as endorsed stocks, bonds and cash letters).”
We tried at three Post Offices and listing the content as “coins” led to a refusal to accept the package. Trying to explain the “Roman” coins were not negotiable, proved fruitless. No doubt professional coin dealers were NEVER intended to be shut down. Obviously, being in that business probably led to some moron calling and being told they were not negotiable under the rules.