Here is exactly what I do and have a 95%+ success rate with fish/shrimp/and snails.
Get a tub of new water and condition it with a dose of prime or amquell plus, something that will detoxify ammonia for as long as possible. Prime is my preferred method.
I also add a bit of nitrofuracin green or methalyne blue to the shipping water.
Generally I use 4x18 2 mil poly bags (I buy from MVP on aquabid, great prices fast shipping)
I put 2/3 water and 1/3 air in the bag, usually filling up about 3" of the bag with water.
Add some sort of plant when shipping shrimp for them to hang on to. Java moss is preferred.
I use airline tubing to put the air into the bag, never breathe into it. Pure oxygen is a better idea but costs a decent amount to use.
Make sure that when the bag is turned on it's side all the shrimp/fish's backs are covered.
Twist and knot the bag, and then either turn it inside out and slip it over again to create a double bag, or add a new bag around the original one upside down so that it cuts down corners.
Wrap In newspaper or paper towel and then set it in the box on it's side to allow more room for horizontal movement.
Fill the rest of the box up with something like newspaper, paper towels, packing nuts, or bubble wrap to keep the bag/s from sliding around too much during transit.
Tape it up and take it directly to the PO.
Creating shipping labels through PayPal is fast and easy, but if you aren't able to weight the box yourself just write the address on and have to p.o. Do it.
Styrofoam lined boxes are preferred but not necessary in weather under 70* and over 86* to keep coolness inside. Also if temps are below or above the mentioned you'll want to add heat or cool packs to the box.