Interesting thread. I used to import fish (had the appropriate USDA APHIS and US F&WS permits and was bringing them back as breeding stock ofr myself or for Stanford University) and used large plastic bags (double bagged) with perhaps 1/3 water and the rest pure oxygen, closed off with rubber bands and placed into a styrofoam container (some are sold for fish transport, a big business worldwide) which is taped shut with shipping information inside and outside, shipped as counter air freight.

I never had a problem, personally, but be thorough and have the required documentation.