Eh, I have gone to the city, bought a fish, wandered the city some more, drove home. No biggy. I also had to move my fish before, they were in bags 6 hours before we actually left... Then when we did they were in the bags for another 1-2 hours before they were put in the tanks.
Bettas are awfully hardy. give them some credit :p If you have one, grab an insulated lunch bag - kid you not. Place the betta and maybe a towel around him/her if you can, in the lunch bag. keeps it warmer then just on the seat.
or grab a kid blanket, or thick towel, and when you have the fish wrap him/her up in the said blanket/towel. :)