Same way it's been done for years. How do you think fish arrive at your LFS? Often, a lot of fish die during the mailing, what you see at your LFS is likely only a portion of what was actually sent.
Usually it's express mail, packed with a heat pack (or cold pack, depending on the climates it will be traveling through), in a well-padded box with water. It does stress the hell out of the fish, but that's why a lot of sites ship only express with a lot of species.