I have had this on a few occasions (as a buyer and as a seller).

Go to the USPS website and file a “Missing Mail” inquiry.

https://www.usps.com/help/missing-mail.htm

I have had 100% success with the package showing up after I did this. I am sure you will get subsequent comments that it did not work.

Either way, it makes some USPS employee look for your package and send you an email about their findings.

It costs nothing. Try it.