Was your critters in ziploc bags? I see a couple in the pics. If they are, they are not appropriate shipping containers and leak. Sometimes when it is seemed via plane, the pressure can make bags that are too full pop.... the water leaks, the box gets wet and you end up with the mess you have.
I'm not saying it's not the fault of USPS, we have no idea what happened along the way, but if the bag popped because of inadequate packaging, the end result would be what you have..... but not the fault of USPS.
Bet he didn't even tell them he was shipping live fish/corals/whatever.