When I received my mudskippers by mail last year, they put them in large fish bags full of air with a small amount of water, and they did put flat pieces of styrofoam in there for the skippers to sit on. It worked great and all of them arrived in great condition. They were shipped overnight express - not priority. Hope this helps! Good luck.
Oh, one thing to add... when shipping fish in general, it's a good idea to add some ammonia remover to the water to help cut down on bio waste. I'm sure it would help inverts the same.