I just thought I'd bump this thread and point out a few things.
First the fish has too much water in the bag, the post does say fill the bag with 1/3 water but the fish in the pic has too much water. Unless you plan on shipping your fish horizontally (laying on their sides, this is how that Thai and Linda Olson ship) only fill your bag 1/3 of the way with water.
Second heat rises so it's smart to place the heat pack wrapped in newspaper on the bottom of the box. Remember heat packs can get up to 150*F so it's extrememely important not to let the heat pack come into contact with the fish bag. But you can also tape it to the box lid like in this post.
Third is you don't want the fish's bag to feel like a balloon, you want to leave it a little squishy so it can expand during shipping.
A good tip I got from the IBC handbook is to place each bag in a brown paper bag and roll it up...this keeps the fish isolated and insulated. You can also roll the bags in news paper.
Here's some good videos on shipping fish:
(Part 1) http://www.youtube.com/watch?v=noITP...eature=related
(Part 2) http://www.youtube.com/watch?v=FHjau...eature=related
(Part 1) http://www.youtube.com/watch?v=Wgmrn...eature=related
(Part 2) http://www.youtube.com/watch?v=hKZKm...eature=related
(Part 3) http://www.youtube.com/watch?v=ICU93...eature=related
*Most of the videos are not for bettas. Remember to adjust the methods of shipment to fit bettas.