This is a method that I have been using with good success for small plants/small trades. It typically costs me between $1.50 - $2.00 in postage. The key is to get the weight below 13 oz. A package can then be shipped with first class postage which is far less expensive than priority mail and typically is just as fast (especially when the trade is only a few states away). I would be careful with this method for cross-country trades which can take a day longer than Priority mail. The plant leaves sometimes have to be carefully flattened and folded...so don't use this method if you want a show-quality speciment right out the box. But the plants don't seem to mind much and quickly readjust their leaves after they've been replanted.
I've set this up in four steps. I hope they are easy to follow:
First I cut a 4" to 6" wide x 16" long (or longer) strip of rigid cardboard out of box. I fold a two inch tab at the top. An then proceed to create folds as shown in this image. Two of the folds must be 3/4" or less apart. The post office charges more if the package is thicker than 3/4".