When I was shipping a lot more often I did several things. I got free USPS Priority Mail boxes (12 x 12 x 8 I believe) delivered to my house through eBay (I used to do a lot of selling on there) and I had a seller who would sell me styro kits designed to fit in those boxes at a reasonable price (maybe $1 a box...precut and ready to go in). I'd use heat packs if needed but preferred to ship during good weather to avoid it. If shipping to Michigan (and maybe some of the surrounding states) I found UPS to be cheaper and faster.
I generally don't feed them for 2 days before shipping. On shipping day (which is always a Monday or Tuesday) I move them into a container before bagging. The stress of netting helps them poop out most of what's left in their digestive system. I still have bag buddies left and use them.
About identifying the contents, what you need to do is write on the box that it contains water. If they don't know what the liquid is they will have to inspect it. Obviously some will and some are lazy and let it go. So if you identify the liquid they will usually let it go (again, some are anal and just gotta see for themselves). This was told to me by the PostMaster of my local office...whom I know pretty well. Also, be sure to have some newspaper to absorb any leakage. If it leaks and the box shows it, it could delay it several days and they might send it back to you.
I also highly recommend kordon breather bags. I've used them several times when shipping fry/juvies and have had good success. You can't seal up the box air tight though, it has to be able to breathe. The breather bags have several benefits besides letting the fish "breathe". You close the bag with no air pocket which takes up less space and lets you put more bags in one shipment or use a smaller box...either way saving you money on shipping. Secondly, since there's no air pocket, the fish don't get swirled around nearly as much during transport. There are also less wrinkles for them to get trapped in.
You should also expect a lost/delayed package or two once in a while...so don't overload your bags. I had one shipment that took 8 days to be delivered. 3 out of the four bags had live fish. The fourth one I'm guessing 1 died and sealed the fate of the rest.
