Chicks ordered to be sent by mail through online retailers (generally through hatcheries, although there are some wonderful small farms who also ship chicks) generally fare quite well. The post office has been doing this since - well at least since my grandparents' would order them in the 1920's for their North Dakota farm! Shipping charges can vary greatly from seller to seller, as well as minimum chick orders. But, the basic premise is that chicks have about a three day period after hatch where they don't need to eat or drink, so they are sent at 1 day old.
I think a very large percentage of BYC members have received chicks via mail, and quite a few have also shipped them!
mypetchicken.com has some great info on their site about the whole process. I'm not advocating for them, I just really loved reading their information when I was somewhat new to chicken ownership.