well, basically we in the EU have had that requirement for quite a while now. That's why - at least here in Germany - we are technically not allowed anymore to send any goods except for paper documents in a simple letter when sending abroad, but have to use a service that automatically includes a customs declaration. That of course meant higher shipping prices.
But time showed, that if you intend to send coins, that are sufficiently light, it still works to send them in a letter, if you pack them in a way that does not make it obvious what is inside and looks and feels like a letter containing paper documents….
For comparison: a regular letter up to 50g sent to anywhere in the world costs me 1.70 EUR. A package with declaration costs me 5.50 EUR within EU and 8.50 EUR anywhere else. So that's some difference, knowing that prices in the US are way different in a negative way…
Plus, there is just not sufficient staff anyway to manually check each and every letter…
And for the declaration, a coin can basically also be described as “metal copper/nickel token”, which is much more specific than “coin” anyway, so is just what the requirement says... :-)
Only “collectable item” might not work anymore, in case anyone at USPS would notice that anyway…
in the end everything will be good - if it's not good, then it's not the end...