I was able to sign up for the 'signature on file' option several years ago. I did this because often times when I check my PO box it is when the retail counter is closed. And like you, it only works about half the time.
This coin took 26 days to get from London, UK, to Miami, Florida. It made it from the UK to the US in three days, then mysteriously sat at Kennedy Airport in New York for nearly a month (or disappeared to God knows where), then magically showed up today at my door with no tracking history as to how it came to end up at my house. When I opened the package I was surprised to see it. I had stopped thinking about it and had totally forgotten it by this point. I guess I just assumed it would never turn up after nearly 2 weeks in New York...so it was quite a surprise to see it here 26 days after it left the UK. C. Licinius L.f. Macer Rome, 84 BCE. AR Denarius (19mm, 3.58g, 6h). Diademed bust of Vejovis l., seen from behind, hurling a thunderbolt. R/ Minerva in quadriga r., brandishing javelin and holding a shield. Crawford 354/1; RBW 1355; RSC Licinia 16. Bankers' marks on obv.
Ah yes, the dreaded ISC New York. We are entering the time of year when it is not unusual for packages to sit there for 10 days or more. Probably because of shenanigans like that, many European sellers are now using Fedex. I wish they all did. I'm glad your coin made it to you safe and sound.
I have to congratulate the USPS on timely deliveries when I buy from @John Anthony - mailed on a Friday and delivered on Monday each time, with tracking showing each scan of the package. However, Canada Post has been a problem. I ordered a Severus II coin from a seller in Winnipeg and it took a month for it to show up. I pretty much had chocked it up as a loss. I do have one quibble about the USPS - a coin from @jamesicus was mailed to me and it never showed up, so he had to send a replacement gift coin.
I had a coin from a vcoins dealer in Canada that took 34 days to show up. Longest waiting period ever for me. I could have probably ridden a bike from Miami to Canada and picked up the coin, and it would have been faster than the mail.
I would NOT use signature on file with your PO box! It is an invitation to have your package stolen and you have no recourse (you "signed"). This happened to me the first time a package was delivered to me and signed for by the post office. I complained to the post office about it and they told me they only investigate claims over I think it was 5 or 10,000 dollars. I basically gave up after talking to them repeatedly and was out $450. I posted a thread about this last year when it happened. John
I have not been able to trace that coin. I mailed it along with three other coins all of which were promptly delivered.