I never tried a heat pack so I wouldn't know but I can say that the cold has never damaged any of my plants. One of my recent shipments was to Canada and it got held up at customs for more than a week. Eventually the box was sent to the person and more than half the plants survived the 2 week journey (obviously not in the best of shape). I think the fact that I used bubble wrap and newspaper around the inside of the box and "inflated" ziploc bags there was sufficient insulation to survive the cold. I've had far worse experiences with the heat of summer than the cold of winter. Not sure what temperatures this box was subject to but being in Canada I'm sure it was quite cold.