I usually wrap the plants in a moist (wet, but not dripping) paper towel. Then put them into a ziplock freezer bag. I'll then wrap this into a plastic grocery bag (need to get rid of those surplus bags
). For normal weather, I'll just ship in priority mail boxes (all priority supplies are free and orderable on the internet). In cold weather, I also insulate the box with a layer of foam. So far this has worked out just fine.