I did order books by U.S. Mail recently with tracking twice they didn't move for seven days and took twenty-eight days to arrive. So anything is possible.
there is a reason USPS "media mail" postage is so low, it is the last mail that moves, every other piece of mail in the GMF gets priority over books and periodicals.
If the USPS is running at less than full ability due to covid, and packages mailed at First Class, Priority. and Express mail rates are waiting to go, they go and your books stay.