Missing In Transit: Antigonos III Tetradrachm

That aint fun, sucks!

Registered mail moves v e r y s l o w l y. Give it at least 30 days before declaring it lost. I'd be concerned but not in a panic mode (yet!).

Not a nice feeling, but "where there's life, there's hope." There is time.
As WS says, the 'Registered' mail system does move very slowly especially country to country.
My usual from the U.S. to Australia is 12 to 14 days, but sometimes up to 35 days, (5 weeks).
Sometimes 'lost' mail has turned up 7 MONTHS later, by the date stamp, but with no further comment?
Fingers crossed for you, TIF. All will be well.

I shipped some coins to Chile once. They got to him 2 Months after I sent them.
I was getting really concerned.
Yeah, give this one more time. You should be ok.

Everyone/anyone: the packages you've had personal experience with (in terms of delays), were any of them *registered* packages?

No I had no idea, I assumed it was lost. It just showed up one day wrapped with US Customs tape without any indication where it was held. Most likely it was at the Port of Entry which is JFK and when they release it, it made it's way here to Washington state.

In my opinion custom clearance is where any letter/package can get stuck, personally had a small package from Europe stuck in Australian customs (at the mail clearance centre) for over 4 months one time. And another time a registered letter for nearly 3 months (and no agency was prepared to say anything).
So I hope yours turns up either soon or eventually (if you were so unlucky that your item got selected for inspection).

I've had a parcel from the US arrive after some 59 days (by the date on the notice I got in the mailbox). I've recently reviewed the tracking number - turned out they still keep the data two years later - and apparently the tracking records it as arriving at my post office less than two weeks after being sent... no idea what happened for the next six (and no wonder it took so long to find when I finally arrived to claim it, a further week later).
I've had another case where the notice date was plausible, but the notice itself didn't enter my mailbox until 27 days later. Was pretty much a mad dash to claim it in the closing post (I've heard somewhere that they only keep parcels for 30 days; I have no idea if that's true, but I didn't want to risk it).
It's all hit-and-miss - other parcels arrived normally (and I've never found out just what was up with my Secret Santa gift last year).

I've heard of a particularly horrific story where someone finally got the notice in their mailbox the day the post office forwarded the item as undelivered. No idea what exactly happened later.