When selling bettas, it is always a good idea to know how to ship your fish.  When shipping a betta, you never want to feed the fish for about 48 hours before shipping.  This will make it so that the fish wont poo as much durring the shipping process.  This is only so that the water doesnt become so polluted that the betta will die. 

Another thing to consider is the ratio of water to air.  1/3 water and 2/3 air is good.  Bettas dont need alot of water when shipping so it saves on shipping cost. 

Bag buddies is a product out there where it will help the stress levels when shipping.

They type of bag doesnt really matter. For best results go with long and tall bags.

i would use a brown box and a styrofoam box.  The bags should be doubled and the fox full of packing peanuts.  Heatpack might be needed if being shipped to hot places. Same with coldpacks.