That is a good question. If properly ventilated, and everything, why not?
So cutting holes in a USPS box is destruction of property? They destroy the boxes themselves!!! Not like many of the boxes get reused anyway, so what's the big deal about adding ventilation if they're still getting postage fees?
I'm going to ask them next time I'm at the P.O. if day old chicks can be shipped in Priority boxes with ventilation holes. I always used chick boxes because they are made for shipping, and even when stacked there are spaces between the boxes for ventilation. I have some Flock-Lock boxes now. I'm not sure I like them as well. They like regular brown boxes, can be stacked thereby reducing ventilation, and you're not supposed to add the excelsior matt or shavings. Seems like a corrugated cardboard matt would be slick for them to stand on? I've used them twice. Nobody complained. I just don't know how well they perform.