The use of breather bags is permissible, but the use of heat packs is not. It is something I have done in the past and it has never caused me any problems.
Personally, I do not like to fish in winter because it can be very difficult for the fish during that time of year. If you want to ship fish in the winter you can probably ship fish in an ice box. I also have that ice-making machine, which is better for making ice to ship fish in a box. As a matter of fact, if I had to use something like this, I would most likely use 3 mil bags, double bagged with a "bag buddy" (Jungle brand probably the only Jungle brand item I would ever use, LOL), and a heat pack inside a Styrofoam.
