I wouldn't risk it personally, but it all depends on the fish. SOme air breather like bichir or lungfish can handle the time, but most wouldn't.
Stop being cheap, fork out the cash to have it ship the proper way. The lesser the time, the better you'll be.
BESIDES, priority mail is AVERAGE 2-4 days. Some had arrive in 1 day, some had arrive 5-6 days...IT'S NOT A GUARANTEE