If your fish barely stay alive on the way home from the lfs you need to shop somewhere else. For smaller fish withhold feeding for 24 hours, larger for 48 hours. 1/3 water, 2/3 air, in a syrofoam box with heat packs for colder weather. You don't want bags sloshing around, either fit the styro to the bag or bags being shipped, or pack the inside so the bag doesn't move. Many shippers use O2 in the bag, breathable bags, bag buddies, or in the case of larger fish a mild sedative. Airport to airport shipping is the best way to get in fish, they are out of the tank for under 12 hours in most cases.
I buy & sell fish at auctions that are bagged for 12+ hours. In colder weather a cooler or styro is needed, a bag or two with hot water & no fish keeps the fish warm in the worst weather. It's -1F where I'm at right now, I would & have transported fish locally in these temperatures.
