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I see nobody is replying to this. I've never shipped tarantulas. There are some good videos on you tube. I don't think the heat packs are anything special. Just an insulated box, heat pack not directly touching the animal.
Im glad you're being sensible and want to keep them warm.
Back in 2008 I had three avic versis shipped from the USA to wales in UK. No heat pack in there. Thought they were dead when they arrived. Containers were cold and they were all curled up. Took them out, room temp wasnt enough to bring them around so i folded a towel up and put in ontop of a heat mat and put them on towel, i swear it took over two hours before the one started to uncurl its legs. I picked them up gently once they started to come around and had them on me for my body heat, it then took another two hours before they were walking around. Needless to say the seller had more than a mouthful of abuse off me