This is how I successfully send plants to my customers, any time of the year

By Nigella

For all plants, a day or two beforehand treat with a good insecticial soap as a precaution.

Woody and Herbaceous plants

1. Remove all soil (Federal and some State regulations, it's embarrassing to have the Ag dept seize your box!). Many plants need to have their soil removed under water, makes things a LOT easier.
2. Soak roots for 15 minutes or so in a transplant shock preventer that contains Vitamin B1
3. Wrap the roots in long fiber spagnum moss thouroughly moistened, then wrung out to the dampness of a wrung out washcloth, with the same solution
4. Bag and tie securely, I prefer wrap tags for tying the bag closed, you can label the tag with a permanent reminder of what the plant is for your trader
5. Wrap with newspaper using the "florist's roll" technique
6. Line the box with a plastic bag (preferably mylar, helps regulate temperatures and keeps humidity up)
7. Stuff box completely with plant rolls and something light, plastic grocery bags work well for this, making very sure there's no "wiggle room" but that plants aren't crushed, either.

Bulbs, dormant rhizomes, corms, etc.

Remove all traces of soil, place in labeled paper bags.

Aquatics

Strong freezer-type ziplock bag (I have found that the zipper types aren't as reliable, I like the new blue Hefty bags but any really strong bag will do) with a small amount of water and the plants, zip most of the way closed, insert straw into the small opening you've left, blow in air to inflate the bag, zip bag closed as you remove straw.

With all of these except the dormant bulbs, etc, you can add protection from heat or cold. In the summer wrap frozen "artificial party ice-cubes" in newspaper, put in bags and tie off (helps regulate temps), In the winter when it's very cold substitute "Hot Hands" packets for the "artificial ice cubes".