Vacuum sealed, lots of bubble wrap, more bubble wrap, and a small amount of bubble wrap. As I read here, if you can't trust it to survive a 10 story fall, you can't trust it with any shipping service. You may also want to wrap a paper towel around the cap to keep it from tearing the vacuum bag.
6 fit great in a medium flat rate box, just don't tell them what's in there... Yeast samples if they push. I've successfully done this countless times.
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