I hate to say it but getting money back from USPS, unless it's a new item purchased from a store with receipt is a *****. They offer insurance but not every package qualifys to be covered with their insurance. That's why DOA guarentee or at least sorting out if there is one or not is a must. I hope they cover yours though.
Thats My biggest fear for buying and shipping fish. that sucks sticky. Keep us posted on how hard it is to get your money back someone may need that info sometime.
If there is no live arrival guarantee, did you get insurance from the carrier? there might be a chance yo might be able to recoup some f our monetary losses from this transaction.