Having read the previous answers,everthing seems to have been explained, I would like to add that it is best to allow light into the box slowly, I have known some fish to actually die from the sudden shock of daylight.
Fish should be starved for two days prior to shipping, this means less pollution in the bag on arrival, and therefore a better chance of the fish living.
As long as you float the bags,add small amounts of tank water,and be patient you should be OK.
We are doing this with thousands of fish at a time,IT WORKS. 8)