Did you know that the “hours” of the heat packs doesn’t only mean that is how many hours it stays warm?
There is a heat curve. The short time period of the heat pack means they will get hotter.
They also only work if they are receiving oxygen. If you seal your box completely, the box will not receive any air and the heat pack will stop working. Let me say this again, DO NOT COMPLETELY SEAL YOUR BOX! It NEEDS oxygen to work. Just like a candle.
As you can see, the shorter hour heat packs the hotter they get. A 20 and 30 hour heat pack can reach over 150°! Yes 150! Whereas a 72 hour heat pack levels out and stays consistent over the time and only reaches around 100°. (give or take)
Either way I don’t think you would want to ship a plant with the inside of the box reaching 150° and cooking your plant.
So just because you buy a shorter hour heat pack doesn’t mean it only stays warm for those hours. You need to look at the temp it will reach during it’s peak.
Also, remember NOT to completely seal your box. IF you are using a styrofoam insulated box you must poke a couple of small holes(pencil size) in the box in order for it to receive air so the pack continues to work.
Keep your plants warm but not cooked!