The biggest danger as tropicfishman mentioned is NOT the oxygen concentration of the water (which will be fine even over long distances as long as the fish is not constantly being stressed), but the toxins that can rapidly build up in the bag/container. That is why I always try to get more water/larger bag even for my short commute and acclimation time (~60min from store to free in the tank).
Remember: Ammonia is a concentration dependent poison, that is, it is near harmless at very low levels that we cannot detect by our test kits, but in higher concentrations (or the same amount of ammonia in a smaller amount of water) can cause irreversible damage. When you stick a fish in a bag which is probably smaller than a 1/2 gallon, think of this as a 1/2 gallon uncycled tank. Now remember the fish is FREAKED out. It was netted, bagged, jostled around, and the temp is probably changing. This causes the fish's metabolism to skyrocket. Just look at all the poop the next time you go to add a fish. It's a huge amount, and predominantly due to all the stress they are going through.
Now imagine all of the ammonia that is building up in that small bag.... (take a small water sample from the bag the next time you go to add a fish in and you will be quite surprised...)
Bottom line, IMO for the best transfer of fish over longer distances:
-Get the largest container possible with the most water and an air pocket (I would sacrifice air space for water IMO).
-Bring along your dechlor product (PRIME in my case) and put in a VERY small amount. Remember this is a small volume. Don't dump a capful in there since that is way over the dosing amount. Normally I'll put in twice the recommended dosing for the VOLUME of water in the bag/container. This will help to detoxify the ammonia that is most definately getting produced.
-Bring along an insulated container or blankets for the transport home. I started the hobby in January and I live in PA. The temp was near freezing during my stocking and so I brought along a styrofoam container lined with newspaper. I purchased the fish, placed them in the styrofoam container, and hit the road. Many fish will gravitate towards the bottom corners of the bag in an attempt to escape (I'm guessing here). This is a pinch point that can harm them if the bag is suddenly moved. So drive easily (pretend your elderly grandmother is in the car).
-When you get home, leave the fish in the container as you prep the room. I turn off all tank lights, loud noises (such as blasting stereo, etc), and keep the light in the room to a minimum. This reduces the stress of the fish.
Then take the bag with fish out and depending on how much water is in the bag do the following:
-If it was a long trip or the fish is large compared to the amount of water its in I would probably do another small dose of dechlor.
-If a large bag/lot of water, dump out ~50%. This has several benefits, chiefly among them allowing space for the tank water your going to add prior to adding the fish, and giving oxygenation to the water, but also allows for slightly faster temp equilibration (the temps should be similar if properly insulated from the LFS), if over longer distances and you know there were very different temps, however, then I would float the bag first without removing any water so the change is more gradual.
-If a small bag/short trip, float the bag.
I give it about 10min, then add in ~25% tank water to the bag, give another 10min, another 25% from the original amount in the bag, 10min, another 25%, then wait a final 10min, dump out as much water as possible (or try to net the fish out but I find this does more harm than good), then dump the fish in smoothly.
I'll normally keep the tank lights off until the next morning since I normally get fish after work ~5-7pm, so there is only 2 hours or so before they would normally go off. This keeps everyone from getting aggressive until the next day and hopefully the new fish has become more comfortable with its surroundings.
The key here is to be as quick as possible in your steps without risking injury/shock to the fish. Note I didn't say the TIME for each step, just how fast you do the step. The steps above sound complicated but when done properly take very little time and make the acclimation to the new tank much smoother. Your fish will thank you for it.