My experience with these things (admittedly small) is that they will survive well in the mail, alive. A diver once mailed me two specimens of another species in a marmite jar of seawater, half-filled. But it came from warm water (Northland). Maybe a specimen collected in cool water might not survive well in a heated area.

When doing specimen collections I take sandwich bags (or some similar zip-lock bag) in my BC. Can transfer to that with ease underwater and then to a specimen jar of some description (ie. washed out marmite jar!!) and post in bubble wrap to Dennis Gordon, NIWA, Greta Point, Wellington.