Keeping them at a good warmish tank temp would be more important if it is cold out or in the car. But they can go through the mail for a couple days somehow and live. Usually a bag on the seat where you would be sitting temp wise would be good.
And there are foot and hand warmers / like aquatic heat packs which could be used if its too coolish. You wrap it and set it next to the bag/heat rising if it is chilly. If the car is warm you wouldn't really need it.
Such a trip shouldn't be an issue. If you have to go inside somewhere and the car is cool you can take them with you inside. I usually have a dark cloth (reusable shopping) bag for transporting any fish I get.