You can ask the store for the largest bag possible, and/or to spread the fish out over more bags than would be typically used. Oxygen would be useful, but many stores only have air, not pure O2. Regarding temperature...I agree that 65F is a bit too cold for fish in a small bag...so don't turn it down that cold! Set the car temp at 75F; compared to the outside 80F+ temps that will still feel comfortable, especially since the air conditioning will also dehumidify the air. Trust me, you'll survive.
When I buy fish on a visit into town I can count on them being in transit for a couple of hours. If I am buying more than just a couple of small ones, I bring a medium-sized insulated cooler with a tight-fitting lid. I have a 1/4-inch hole drilled in the lid and run an airline and airstone into the cooler, fed from either a battery powered pump or a standard 120v pump plugged into a small automobile inverter plugged into the cigarette lighter. I use the same set-up when bringing fish I have bred into town for trade.
Important to remember that the cooler needs to be at room temperature before the store fills it with water; don't leave it in a scorching hot car in the sun for hours, and then dump in your fish. Also, the air pump will be drawing air from inside the car and pumping it into the cooler, so again, try to keep the car more or less at the correct temperature.
