For shipping plants in general this is what I do.
Pick up a USPS Priority Box - 7"x7"x6", you can get this for free by ordering at least 10 online at
www.USPS.com. Next you'll want some paper towel (plain white is best) and a plastic baggie (I prefer the ones with the zipper) big enough to hold the moss. Dip the paper towel in your aquarium water to get it damp (completely wet through but not sopping wet). Then wrap your moss in the paper towel and place it in the baggie. Get as much of the air out of the baggie without risking harming the plant and seal the baggie. Use packing peanuts, crumpled newspaper, etc to fill the box placing the baggie at the center. This will help keep the plant insulated and keep it from getting banged up during shipping. Mail via USPS Priority mail, which should only cost $4.05 if you kept the box under 1lb. The package will arrive in 2-3 days and as long as the temps where good, the plant should be in excellant condition. If you purchase the shipping online at
www.USPS.com you get the Delivery Confirmation for free.
Good Luck!