I get a bike now and then from a guy in Texas. He cuts the end off a bike box about 12" in, and then tapes up the large portion and stands it on end. Then he takes the brakes off the mounts and the chain rings and pedals off. He also removes the fork. He slides the rear wheel/frame down into the box, then the bits, then puts in the front wheel and gets cardboard/foam etc between touching parts. Then he slides the end he cut off over the other part. The box is then about a foot shorter and meets USPS rules. He's shipping for about $20.
He's shipped me two or three with no damage.

I tried it with a bike to Ohio from NC forBike Tales. < $19.00 shipped via the Post Office.