Replying to Payment question

earlier this winter i bought a track jacket from some kid from scranton. i mailed him 20 bones because i didnt have paypal. the envolope never reached him because either someone stole it or i just got lost. now he wants me to send him 20 more dollars. i dont think this is fair because i sent him the money and it isnt really my fault it got stolen or whatever. I have good feedback and i swear i mailed the cash. how should i deal with this problem?