While I haven't shipped fish before, I've received plenty via mail.
Bill's questions are all very relative to what you need to do.
For starters, I'd say start with 1/2 of your water in the bag to be tank water and 1/2 to be fresh dechlor'ed (or aged water). Add either a Bag Buddy or a piece of that denitrator filter padding. Total bag volume should consist of 1/3 water, 2/3 air. Go to the LFS and get them to fill the bag with straight O2. They won't mind.
For 1" fish, you can do a bunch in a bag, for 2" fish only couple per bag and larger than that, go singly. Double bag using the thicker bags and do not use breathers... for spiny or bitey fish, you may have to get creative or use puncture-resistant bags.
Use a styro cooler that has a box that matches it. I have a few extras after our food order, but will be using them to bring for folks at the meeting. How many bags are you packing? I have a few large ones and a pretty sweet smaller one (2.5' x 1' x 8" high approximately) that may work out well for you.
Not sure if you need ice packs or not, but I have plenty of those as well. use newspaper to prevent the bags from rolling around as well as to keep space between the bag and the cold packs.
