What do I have to do to ship a plant via the United States Postal Service? How do I prepare a plant for mailing? I want to mail a one gallon plant.
When sending your one gallon plant, you should mark the box Fragile and Living Plant Material. You should also use the fastest shipping method you can afford, to make sure the plant reaches its destination in good condition. You might also phone your local post office to learn whether there are any other regulations concerning mailing live plants. The U.S. Postal Service provides these basic guidelines for packaging perishable plant matter. Another matter to consider is whether plant quarantine regulations apply to the plant you are shipping, and whether or not you need a permit.
FedEx has a guide has a guide entitled Packaging Guidelines for Flowers and Plants which should help you.
In terms of packing the plant, you should also ensure that the plant does not move around in its box during transport. Avent (see the source below) recommends securing the pot to the side of the box, with strapping or similar material. You may also want to protect the vegetation with breathable material–perhaps biodegradable starch-based packing peanuts?
Source: So You Want to Start a Nursery, T. Avent, 2003, pp. 75-78