What I don't understand is that these boxes have tracking numbers and if they don't show up you can locate then immediately. If you have the tracking number, you just go to USPS online and plug in the number and it will give you every way point the shipment has passed. IF and this is the thing IF someone does run the number. It should be a simple thing to have them locate where the box went astray and have the animals given water at least. If postal employees screw up and a live animal is delayed in reaching it's destination, then the PO should be required to provide the basic care to keep them alive until they can be delivered.
Of course, if this is brought up and looks like it might come to be, then the PO will simply say they can no longer deliver live animals