Be aware that ammonia and carbon dioxide will build up. Because carbon dioxide is acidic (it forms carbonic acid when it combines with water), it lowers the pH, and shifts the ammonia/ammonium ion equilibrium away from ammonia. So carbon dioxide buildup actually helps keep the fish from dying of ammonia poisoning.
However, if you get it home and immediately dump a bunch of aquarium water into the bag, the pH will return to about 8.3, and the fish will get a blast of ammonia.
So it is better to transfer the fish to new water before acclimating it (i.e., acclimation does more harm than good if there is ammonia buildup in the bag).
Take a styrofoam container with you to put the bag(s) in. That will help minimize temperature fluctuations.