It also depends greatly on the type of fish you are shipping. Some fish can handle it just fine, other fish I'd wait until warmer weather. I have used the breather bags and used the regular bags, and I don't see much difference. I can use much less water with breather bags (not for use with labyrinth fish) and save on shipping that way.
If you have a relatively hardy fish, the very best insulation is... I'm not kidding... household R-19 insulation, like the kind you put between the rafters in your attic. Owens Corning Pink Panther type, you know the stuff. Wrap your bags in that and stuff it in a cardboard box and you are good to go. I have a lot of experience using it and I gotta say it is better than styro. I'd still use a heat pack this time of year, though, depending on how close you are to the equator.