I've been selling - and therefore shipping - around the world for 16 years. And I'm always amazed at complaints about USPS.
We lived in North Carolina for 3 of those years - where FedEx wouldn't even answer the phone or return calls. Where UPS was extremely unreliable.
In my 16 years, a $100 engine supposedly wasn't delivered to a buyer in the Ukraine, a $6 part supposedly didn't arrive in Puerto Rico (despite USPS delivery confirmation), and two packages went to the wrong city initially. Golly gee whiz guys, can you pleeeez point me to a better service?
I suspect most of the problems can be attributed to a carrier along the way, but that's only my guess. I did deliver mail as a substitute for a time, and can certainly state that carriers differ as much as any other group of people ...
One of the things I learned early in life is there is almost always a chain of command. Use it!
I have used for all of those years a First Class Package process with free delivery confirmation. I verified the correctness with USPS national customer service, so was surprised when we moved to another location and I was told by a clerk they couldn't accept my packages as prepared. I asked the clerk to check with the Postmaster; she returned with the same answer. Then I asked for the name and phone number of the regional supervisor. After another chat with the Postmaster, she returned to say they would accept the packages for me. Gosh, I'm special! Or, willing to go up the chain?
One thing I can say from my own experience: the Postmaster I worked for absolutely hated to hear from above!