According to USPS, the most important thing we should do is either place a photo copy of the shipping label or hand write a copy of the addresses of both the shipper and the intended receiver and the tracking #, inside the box.
If the original label comes off, or gets damaged and unreadable, etc., the package is sent to a USPS recovery location. Once the package arrives, it is opened to determine what the package contents are and look for shipping information. If all the shipping information is included inside the box, they will recreate the shipping label and reship at no additional expense.
I found out the hard way...