i advise against using heat packs 99% of the time.
the package normally isnt left outdoors in a blizzard, and you can cook whatever is in the box if you mess anything up in the packaging, or if the box is too small.
i have never used a heat pack with plants, and dont remember ever getting a single DOA plant (i did once have a very brittle stem that got tossed around bad and broken into 1" segments, but thats unrelated).
Eheim Pimp #296
Canon Pimp #67