If they're packed properly (double-bagged, heat pack if necessary, packing peanuts or similar, styrofoam box inside a cardboard carton) and sent via a reliable air express service, you should have no problems.
Bettas I've had shipped to me via air express courier (within Australia) have arrived within 24 hours, and were as lively and fresh-looking as when they were sent. How long the fish are in transit obviously depends on the distance involved and how good the courier service is. Up to 48 hours is okay, I think, but personally I wouldn't want them to be in transit any longer than that,.