I'd say that losing the top is not that bad for the tree. If the root system is strong then it will re-grow the top with 90% probability. On the plus side, that top that was knocked off can be used for cuttings to start some new trees.
I have shipped a number of trees in the usps priority triangular mailing tubes. The shipping cost depends on the weight, but you don't get charged extra for the length. If you bring your own box, they will measure it and may charge extra if it's over a certain length, width, or height. Also, the box itself is free, which is a pretty good deal. Another good thing about using priority mail is that it comes with $50 insurance for no additional cost, or you could purchase more for what would probably be a relatively small fee.
Whatever you do, tape up the root mass or pot so that nothing comes loose, and moisture is retained. Don't completely seal it. Leave/cut a few holes so that it can breath. I like to put it in a plastic grocery bag and tape that up. That way it's easier to cut it free when they get it.
Is it a heavy infestation of ants, as in a nest in the pot, or just a few? If it's just a few, I wouldn't worry about them, as long as they're not fire ants. If they are fire ants, I'd get rid of them first. There are eco-friendly ways of doing this, I think, but right now I'm outside of the fire ant zone, so I don't recall.