If you can fit the package into a USPS triangular Priority Mail tube (they are 36 inches long), it should cost much much less than what you quote above. I have mailed a dozen or so 2 gallon trees all over the country, including from Maryland to California, and I think the most expensive one was around $15, with the average being about $8. I have shipped on Saturday and every one has arrived on Monday, no matter where in the US it was going. Sometimes they don't have the boxes out front and you have to ask them to go in the back to get them.
Now I usually slide mine out of their pots and knock off excess soil, then smush them into the box, which I wouldn't necessarily recommend for tender-rooted airlayers. But you could plan ahead when you are starting the airlayer such that they should fit into such a box. Just tape 4 pieces of plastic together to make a triangular temporary pot, for example. Or use one or two pieces of plastic bent into the right shape.
The key here is that you are using the USPS box so:
a) you don't have to pay for a box and
b) they don't charge you extra for irregular dimensions.
Some folks disagree with my semi-bare root method, saying it will hurt the tree, but I say why pay to ship dirt or water across the country? That can get expensive. However, I agree with them for an airlayer, you should definitely have some way of protecting the tender root ball. If you don't you may as well just send a cutting instead. So you just have to make sure you set it up so you are not using much soil/mix/sphagnum/whatever, because you shouldn't need that much for an airlayer anyway. Certainly not anywhere near 10 lbs. More like 1 or 2 lbs.
Also, those weights do sound extremely high for the size of the pots. What were you using for the mix, some sort of heavy dirt? Or did you saturate the mix with water before sending it? My 2 gallon trees usually weight around 4-8 lbs, box included, with some of the media knocked off.
I'll bet you could even set up an airlayer that would fit inside one of the priority mail flat rate large boxes. That would be very cheap to send.
The only other thing I can think of is that maybe they charged you for a second day or next day air rate. It shouldn't be that much for ground mail.
You can go online and put in your dimensions and the destination zip code and see what the cost should be.