Idk if you have seen the urine container thread? But when shipping a single polyp the urine container would be great and the small box can be used and should cut down shipping costs by a lot.
IMO yes there are cheaper ways to ship...... But coral's are fragile and i dont think they should ever be shipped anything but priorty overnight.. Like Gerbe77r said the smaller the box the cheaper the shipping charge, you can also have the package held at the station for pick up ( this saves a few bucks), they also charge extra for any insurance you add to the pkg and if you select a direct sig/ indirect sig they charge for it. How far are you shipping the frags? How close does the person live to the delivery station/terminal/hub whatever they call it? All these will also play a factor in the shipping price.
You can ship zoas through priority mail. Use a well insulated box with a cold pack and you are good to go. We have a couple customers that we ship coral to through priority mail during the warmer months. Never have once had a problem. But, you want to make sure that the coral is packaged so that there is no way that the bag can leak, take the package to the post office as late as possible.
If it's an expensive purchase, it's probably not worth it to save a couple bucks just in case there was a delay.
With that being said, Zoanthids (and most other Soft Corals) are very hardy and IMO wouldn't have an issue providing the weather isn't too extreme.
PS- Package dimensions and weight obviously play a huge role in rates. If you can fit a frag inside of a USPS Priority Flat Rate box, it shouldn't cost much (~$15).
I would bet that you could send a small frag in a tiny pouch of water in an express mail envelope for like $17. It would be overnite and cheap just couldn't have a lot of weight so a tiny amount of water and you would want to ship when the temps are around 80 degrees in both areas. You might even be able to get away with some insulation material in there if the weight was not over. I ship macro algae like that in wet paper towels. hth