Shipping a rifle out of state must go to a ffl. I have never had to show or sign anything at the post office saying it was unloaded. I would get signature confirmation as well.
ETA: I would also not have anything firearm related written on outside of box, and get insurance. Other than that, you should be good to go. I just pack mine up nice in a plain box, walk in, tell them it's a rifle (not sure if you have to), get delivery confirmation, insurance, signature confirmation, and ship to a FFL. Never had a problem, and I've shipped a dozen or so.