GaleForce,
I spoke to our local postmaster, and as long as the box is marked 'Perishable', jic it ends up in the 'dead maill' area, and they know to toss it, so it doesn't stink up the place, we are ok. If there is a LOT of milk, you need to mark it, 'contains liquid', also jic of spills, so they know it's not a toxic substance.
I mark the outer baggie,
Kefir 'yeast' grains in whole milk, so if it is opened, they will know it's harmless and not a toxic substance (or some other funny stuff).
If going 'across the border' I mark the customs slip,
'Kefir Sourdough Yeast Starter' - Perishable' and that way customs officials will know it's not some toxic or other wierd stuff (I didn't figure many of them would know what Kefir grains are, so I just called it a sourdough starter as most folks know what bakers yeast is.
So far, this complies with shipping regulations. Hope this info helps anyone wanting to ship Kefir grains and was worried about all the regulations they have now days.

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