i've not personally bought any fish thru the post (i prefer to see them in the flesh first) but i've bought quite a few corals.
They tend to come in a plastic tub wrapped in filter wool, with a heat pack sometimes used to try and keep the heat up.
I've had zoas, mushrooms and trumpet corals all posted to me with no casualties.
The last frag of zoas was left at the post office for a second night as i didn't have enough time to collect it that day. The water was stone cold by the time i collected it, but after a lengthy acclimatisation they all opened up fine.
I have just cut my own zoa frags so in the near future will be sending my first ones out thru the post using the above methods.