In addition to shipping enough chicks for warmth (or with a heat pad), you have to ship chicks as day olds so that they can survive without food/water for 2 days. Otherwise, you have to wait until they are completely feathered out and ship them in adult shippers with biofilters.
That being said, it is typically safe to ship chicks as day old or again around 6-8 weeks old (and older). I would highly recommend using only Express shipping. Priority has no guarantee and while I know hatcheries use it, they can also easily refund or replace birds, which breeders cannot do so as easily. Express has a 2 day guarantee (sometimes next day), so it would ensure fast arrival and healthy birds.
That being said, it is typically safe to ship chicks as day old or again around 6-8 weeks old (and older). I would highly recommend using only Express shipping. Priority has no guarantee and while I know hatcheries use it, they can also easily refund or replace birds, which breeders cannot do so as easily. Express has a 2 day guarantee (sometimes next day), so it would ensure fast arrival and healthy birds.