This is what I do:
First I make a sugar water solution for the grains. I use 1 cup of water, 2 tsp sugar, and 2-4 Tbsp water crystals. I got the ratio off Dom's Kefir Insite: http://users.chariot.net.au/~dna/sha...fir-grains.htm
I then pour the solution into a quart sized freezer bag and seal it well.
I then place the bag in one of those "disposable" tupperware type containers, the kind you can get at the grocery store in the asile with zip-lock baggies and such. i think they are $4 for a pack of five. I like using these as it adds a bit of rigidity to the package, and the ziplock bag can't get squashed and rupture. And if there are leaks in the bag they should be contained by the tupperware.
I then put the container in a Priority shipping box (there is a small one that fits the disposable tupperware I buy perfectly, though some brands do run a bit too big for the box), attach the lable, and ship away!