Originally I was going to use 1" thick styrofoam insulation in my shipping boxes but now I'm debating whether I should use the styro or the reflective foil isualtion...it's more expensive.
Here's what Bettysplendens says about it:
As of January 2010 I have switched entirely to reflective foil insulation in my shipping boxes. This material was first developed by NASA to protect the astronauts from extreme temperature shifts. It works by reflecting potentially dangerous heat from the outside as well as retaining the heat on the inside, thereby protecting your bettas from extremes of both hot and cold. It is more expensive than the styrofoam board I have used in the past, but I've had such excellent results with it that I consider it worth the added cost.