With USPS it's 100% legal to ship live fish and liquids provided they are packed correctly. However, I do not think it's anyone's concern what's in the box. You can say anything you want really. I doubt they'll shake the box or anything. The problem with telling them there's fish in the box is that they'll put the fish near an AC or heat vent depending on weather or cause delays if the workers are not familiar with policy. Most transhippers leave their boxes blank.