I find that expelling the air works for short trips... great for taking plants in a suitcase... and for up to a few days in transit... but some plants dont fare well for much longer. You never know when the 2-day priority mail will take 5. I like to ship my plants - emersed or submersed grown in a plastic bag with air space..or in a take-out stryo like you find on the salad bar enclosed on a plastic bag (also from the supermarket).... in either case, only with a little moisture in side. As long as the plants are in 100 % enclosed humid environment, they will do fine.
For the flat food styros, I line them with 1 sheet of newspaper or old phone book (thats how i recycle mine). The moisture they soak up is sufficient. I shake out the extra water. These take out containers fit into the free Priority mail boxes. This way, you can ship a lot of plants for under the 1 pound charge of $4.80.