docsold, it sounds like there were a couple things you might have done differently to improve their chances.
First of all, I only ever ship on Mondays or Tuesdays when it comes to fish. It's a reliable way to ensure quickest transit time for the fish. Shipping over the weekend can be iffy.
Secondly, I agree with David - using a dechlorinator/ammonia remover and pure oxygen in the bags is an excellent method if you have access to that. In the past, I have talked with my LFS and they let me stop at their place before sealing up the box. Open up the plastic bags one more time, let out the air and replace with pure oxygen from the LFS's canisters. Then seal up the box and head directly to the post office. It's much cheaper than buying your own oxygen canister and easier as well, since the LFS will sometimes help you finish packing up the fish.
Third, Never Ever Ever send fish priority mail. 2-3 days is the USPS estimate, but no guarantee. I have had "priority" packages arrive 7 days after their mail date. Express mail is always better.
Finally, you can claim losses on fish but the package must be labeled as such. Put "LIVE FISH" in giant letters on every side of the box, use UP/DOWN arrows, and affix a label near the "to" address reading:
PERISHABLE
Small Cold-Blooded Animals
DMM CO22.3.2
"PERISHABLE" should be in 22 point bold type, in red ink. The DMM number will refer the post office employees to their Rules and Regulations manual, right to the section that deals with shipping live fish. They will try to get out of giving you money back if they can, and if the package doesn't have their regulation label on it, they can do that.
Best of luck in the future!