u/the_disintegrator anwered the question, but I will add...
I have learned the hard way that the USPS operates on a "best efforts" basis. If a package disappears there is not a damned thing they will do about it unless it is insured, but if it is first class (even with tracking)and it stops tracking and then is never seen again... the USPS will say "oh well" and they do not have to do a thing.
With that said I have never (yet) had a package disappear and never reappear at some point. I have seen a couple go missing for 2-3 weeks and then suddenly show up... and no one know why.
I do recall Newman and his (Jerry's) storage unit stuffed with mail.
One time I got a package 3 months late. Many times I have had a package go missing for 10-15 days.
The first class tracking is awesome when it is delivered and things go well... but when the package goes missing the tracking is worthless.
Here is the bottom line. (I am reminding myself here) No matter what NEVER yell at the post office people even when they are dead eyed, could not care less, anti-help, walking dead zombies. Yelling at the post office people will solve nothing and will probably cause other problems (I wont get in to what those problems can be).
Always talk soothing and smile with both the buyers and the post office people. The more I wanna blow my stack... EXPLODE!!! SICK OF IT!!! DONE WITH THIS BS!!!... the more calm I act. I say things like Okey dokey, things happen, it will all work out, I am sure it will be fine, lets wait a few more days, no problem, it will be fine
Bottom bottom line- From the start operate like you know the package will going missing. Either self insure (be ready to refund money or send another item) or pay the insurance and accept it.
And I always take my package up to the counter person and let them check it in.
shudder as I (waste my time) standing in the molasses line and see person after person walk in, dump their packages on the counter and walk out, when the post office person is in back, and the packages just lay there.. and lay there. There is nothing stopping anyone from picking the packages up on the way out.... but.. I am sure it is a fine way to do things, it will all work out for them... okey dokey.