Just tell the store to double bag it and that your going to bring it abroad. make sure they put enough water so that even if the bag is sideways, the fish will still be okay. just get the plastic and put it in a shoebox and check that in. i did it and it was fine. just make sure your bag isnt so tightly packed.
i have not had any casualties so far. i brought on separate occasions a purple tang, a hawaiian flame wrasse, 2 helfrich firefish, flame angels.
if i had casualties, its usually months after, like when a fish goes missing for no reason or just stops eating.