If the plants are in 4 inch pots and the plants aren't taller than 4 inches, what I would do is stuff some paper towel around the plant and over the soil and tape that down tight so that no soil can get loose and it haolds the plant down. You can take another 4 inch pot and tape that to the other one which will give you a nice protection. You could also just wrap some paper around the top of the pot and tape it to that if you think the double pot method is too much. I would then tape the pots to the box on the inside so it doesn't matter whether they turn it upside down or not. If they aren't in 4 inch pots you can just wrap the soil and the plant in a paper tube type of thing and just tape those to each other and also to the box. If you ever have taller plants, the PO people have these great boxes that are in a triangle shape and I can ship 24 inch tall plant in those very well. What I do with cycads is to bare root the plants, wrap the roots in slightly moist sphagnum moss and wrap the wad in aluminum foil. I can then tape the foil to the triangle box and no need to fill with peanuts or anything, you can throw that box, turn it upside down and that plant isn't going anywhere. Tom