They do make large Priority Mail Flat Rate shoeboxes, that should hold a boot box, depending on how big they are.
Hi! I just shipped out a few pairs of tall riding boots in the last few weeks in the original Tory Burch boxes. I was able to get heavy brown shipping paper at Home Depot and wrapped the original box and sent it UPS. They got there fine! I did state that I was using the original box to ship in the listing.