I don't give a damn about someone "thinking" about doing something he shouldn't do, as long as he didn't do it. All of you who've never done that are welcome to pile on. The rest of you (that would be everyone) should probably drop the self-righteous BS.
OP - Your knife was delivered to a box owned by Robert Reynolds. It was signed for by Robert Reynolds. Jonakee Reynolds asked you to send the knife to that box. I assume that you have correspondence of some kind that lays out the terms of your deal. I would do these things, in this order: Call the La Madera post office and tell them what's going on. (Btw - La Madera has a population of fewer than 200 people. The chances are good that the postal workers know Reynolds and would remember this transaction). Ask them the procedure for collecting a parcel mailed to one of their boxes. Ask them if they have surveillance cameras. Assuming you are satisfied that there is no question about who picked up the knife, call Robert Reynolds. If he answers, ask him where your knife is and what his plans are for returning it to you. If you get no answer, leave a message that says exactly what happened and that your next calls will be to the Rio Arriba Sheriff's Office and the US Postal Inspector. If you don't hear back within a few hours, call the Rio Arriba Sheriff's Office.