This is exactly what I was going to suggest. Comfrey root cuttings are super resilient anyway so not much to worry about.
Discussion
We have shipped over 10,000 root this way. Just had an order get stuck in the mail system and took 13 days to arrive. They appeared to still be viable. (We will replace them if they turn out to be duds but they were still firm and didn't smell per the customer communications)