Shipping big fish is a little more complicated because they need a much larger water volume and large amount of air. It's best done in a medium or large flat rate box so weight is irrelevant.
I've shipped fully grown adult angelfish this way via priority without any problem, I just insulated the box, then figured out how big I could make the bag, more air the better.
I just make sure the water covers the fish completely no matter which way the bag turns, add a few drops of prime and as much air as you can fit in there.