Appears that I am experiencing my first lost USPS package. Spent over an hour today at the local USPS. Without getting into major detail on the lost Medium Flat Rate box, the best guess is that the label was damaged enough that the barcode, return address and ship to address were no longer readable. When I check the tracking information on the USPS website, the package never shows received by them.
The USPS supervisor told me that when a label is damaged enough that the barcode, return address and ship to address were no longer readable, the package goes to a location called "Dead Letter"
Once a package moves to the "Dead Letter" location, the package is opened to see if the barcode, return address or ship to address are noted inside the package. If they are not, the package is destroyed by USPS
Definitely learned an expensive lesson to the tune of $200 In the future, every box that I ship via USPS will have a copy of the shipping label packed inside the box. Please learn from my mistake so this doesn't happen to you.