when it is hot here I dont put in heat packs. Heat kills quicker than cold (usually). When I did herduccks and BM6's box I did no heat packs for that fear. The box was packed with babies though, and had foam pads in the floor for insulation. For two I might add it to one end of the box so they can move towards and away from the heat source in the box. I like to place in the floor under the flap or make a false wall on the side with cardboard and tape the heat pack there.
It looks like the low there will be 19°. The boys have only experienced 30° here, but they did just fine. What will happen if the box is a hair over 8 lbs? Is the USPS that strict or will they allow an ounce or 2?