be sure and check with your post office first, Nick...Hawaii can be somewhat crazy about accepting what they deem to be "plants"...especially in this day and age. Too bad really...the days of free exchange seem to be fading away... perhaps if you told them what it was for, they might wave it?
(not meaning to throw a rock into this train track, just have had some bad experiences with shipping animals and plants lately...)
Regarding the legality...the post office and ups inspect all packages for live cargo(plants and animals). If it was contraband, the shipper could be fined heavily...ask the guy who brought in the asian arowanas by mail...