^ what he said.
see if you can get a guarantee too.. it might cost a little extra, but if you can guarantee that they'll pay for it if they do something wrong, it would be a lot of weight off your mind. also, consider putting a styrofoam box inside another box with the bag of fish in the very center. the styrofoam will help "absorb" some of the forces that might shake the fish up.
also, they make gas permeable fish bags, so that oxygen and whatnot can still get in, but the water can't get out. this would be a good option so you can fill the whole bag with water (meaning the fish dont get sloshed around as easily)