you just get a regular white envelope and put him in it, put the stamp on and drop it in the mailbox.........
I'M KIDDING!!! i'm not totally sure how to ship a betta, but he needs to be in a container or sealed bag of water, probably surrounded by lots of padding, in a box... perhaps someone who has actually shipped a betta can tell you more..
you need to get a styrofoam cooler and pack it with newspaper or something else to cushion the betta. put the betta in a shipping bag with water to cover just a little bit over his body (about 1 1/2 inches to 2 inches of water) and you need to fill the rest of the bag with air. (dont blow into it to fill it up-then youd be filling it with carbon dioxide). you also need to put some methylen blue, polyaqua or something made for shipping fish in the bag to keep the betta from getting too stressed. put the cooler in a box that fits snuggly around it.
i would recommend shipping the betta overnight which is about $25.
and be sure to mark"fragile" and "this side up ^" on all sides of the box.
Bettas are more hardy than most fish and their labarynth gland helps them through most stressful situations... I think I would rather have the betta in 2 - 3" of air and at least twice as much air.
well the betta needs air while being shipped seeing how it breaths above water, that is the main reason. also, for the betta, how much are you looking for him?