When you need to reach a person at the USPS you can call your local post office with their local phone number.
Then they can get you in touch with someone that you need to talk too.
Sometimes you can solve issues by talking to your local post office and post master which I have done before.
The Toll Free USPS number for customer service is 1 (800) 275-8777
You can call that number and they can route you to someone to speak too.
But you can also reach someone through your local post office and have them help you solve issues such as issues with shipping and tracking or a lost package.
The local post office has more ways of seeing where your package is than normal tracking can provide.
So if your package has not shown up when it says it has or it seems lost then contact either your local post office or the 1800 number and provide them with your tracking number.