I recently read on the Board of Inquiry about a person who bought snakes shipped by a fairly well-known breeder (should have checked the BOI!!!!!!!!!!!!!!!) who shipped second day, and the snakes died before arrival.
It is in the contract a certified reptile shipper signs that they will only ship overnight.
If a certified shipper's package gets lost/delayed/whatever, they can call FedEx and say, hey, I have live animals in the package, it is extremely important that you locate the shipment and move it. If someone is sneaking live animals, they can't do that. I recently read of someone who had a package delayed overnight, and a kind person at FedEx opened the package, by her instruction, and watered the snakes. That is above and beyond- but if you were sneaking snakes, that would never happen...