A couple of things come to mind to cut costs, some already mentioned. Sorry if repeating but I support all these ideas:
1. Cut down the height of the tree so that it fits in a smaller container.
2. Wait until tree is dormant and remove the bulk of soil, replacing the void with moist wood chips or paper (I do not advocate complete bare rooting a tree - used to do that and some did not survive).
3. Use the USPS triangular tubes if possible (use the 25.5 inch not the 38 inch tubes). I have actually squeezed a one gallon pot in one, cutting the upper rim off the one gallon container- it will bulge one end of the container but once taped good, it works!
4. Of course, make smaller air layers. 
5. Consider UPS or Fedex as other carrier options.
6. When making the air layer, consider sending it right away to the buyer instead of up potting it and creating a heavy problem. It you remove all but the distal 2 or 3 leaves or cut them in half, the A/L will survive over a few days of travel.

Leon and others, these are all good suggestions.  I like the idea of sending the airlayer in its original container (rather than potting it up and sending it later).  If the airlayer has a good root mass, and all but a few leaves removed about a week before it is removed, it should be fine.

I do ship trees.  Sometimes and depending on the tree, I will remove all soil and ship the tree with root in Watersaver gel cubes.  It looks like clear jello.  All you need is a very small amount of gel.

Dennis,

Interesting idea.  I'm not able to find "Watersaver Gell Cubes"

But did find this.

Is this the same product or were you thinking something different?