Look for styrofoam at a home improvement store. They sell 3/4" sheets that are about 13.5" X 48" for putting between wall studs.
I start with a 12X12X12 box. Cut the Styrofoam to fit tight on bottom and sides. Leave the sides 3/4" short to accommodate the top piece of Styrofoam. Double bag with about 1/3 water, 2/3 air. I don' t use oxygen, but some do. I also don't use bag buddies, but I do add a couple of drops of Ammolock to detox ammonia. I don't feed the fish for 24-48 hours before bagging, but they will still excrete some ammonia. There's no filter in that bag, so it's inevitable that some level of ammonia will exist by the time they reach their destination.
I use shredded newspaper for cushioning along with a layer of bubble wrap around the bags. Tape a 40-60 hour heat pack to the inside of the top piece of Styrofoam, if temps require it. Lots of tapes won't stick to Styrofoam, so I use 'gorilla' tape that I also found at a home improvement store.
I'd recommend USPS Express service. They've recently gone to a zone system for pricing, so depending on distance it may be more or less than it used to be. Ranges anywhere from about $20 to about $45 for a 6-7# box. Check out usps.com to get service commitments. They guarantee next day to many zips, but not all. Nice thing is that if they don't meed the delivery commitment, they refund your money. I've used them 8-10 times now, and they've always met the delivery commitment time. Also, no DOA's yet either. Print your label online and you'll get 3% off the cost. You just need to find an accurate scale first. Anything over an even pound, even a half ounce, gets charged the next higher weight.
I also don't ship other than Mon-Wed. You don't want your package tied up over the weekend if there's a problem.
HTH
