He should be OK. He's sitting at the post office, which is climate controlled.
(If he was sitting outside, in the middle of a snow storm, or a heat wave, that would be far more dangerous!)
You'll need to acclimate SLOWLY once you get him. Since he'll be spending a few extra days in the bag, the ammonia level will be a little higher.... Just go extra slow on the acclimation, to give him time to adjust to your water parameters.
He shouldn't need any aquarium salt.
Use either the Stress Coat or the Prime, whichever you prefer.
I'm thinking you know how to acclimate, right? If you're not sure, just ask, and we'll explain the process.
If he looks stressed, keep the tank in a quiet location. Cover it with a towel or sheet. Bettas tend to calm down when they're in the dark.
(Whenever I get a new betta, I acclimate over the course of at least 4-to-8 hours. And I keep the tank in a quiet, dark location during the process.)
Good luck! Take pictures when you get him!