I have only shipped adult birds who can go without food and water for 2 days.
The post office was slow and it took 3 days.
I don't know many chicks that can live that long without food and water.
The reason they ship day old chicks is because they can go a couple days without food and water because the mom sits on the eggs til all the eggs hatch and that takes a couple days, so chicks are designed to begin eating after a day or two when every chick in the nest is hatched and ready to go.
But juvenile chickens are more fragile. The hatcheries ship them at about 10 weeks I noticed if you order juveniles. I think from hatching til 10 weeks, they need the food and water consistantly.