Kombucha: The key is to package it with a half cup of tea and seal it really well. I have equipment to do it since we work in bulk and need them to be tamper evident but you can do essentially the same thing with heavy duty zip lock bags. I agree with the PP though to triple bag--the post office has this thing about leaky packages
Kefir Grains: They can be shipped fresh or dried. If shipping fresh you want to pack them in liquid (for water kefir grains make a sugar water solution of 1/4 cup sugar per quart of water--less sugar than usual to keep the gas level low so the bag doesn't burst; for milk kefir, ship in fresh milk). Ship the same way you do Kombucha but be sure to pick a quick shipping method--water kefir grains in particular won't last too long like that (milk kefir is a bit hardier). To dry the grains, rinse thoroughly with filtered water and then lay them on some unbleached parchment paper in a safe place for 2-5 days. When they are completely dry, just package them up. They are generally shelf stable for at least 6 months as long as they aren't exposed to excessive heat.