I had the same problem with this crazy lady from cali!! OMG she was such a *****!
They do this so they can get the bag for free, its a total scam, at first they act all innocent, and say "omg the box is empty" they will ask for a refund or if you can send some thing else that you sell..I hate scammers..
When this happened to me I freaked out, I actually belived her at first.. then they make a sob story so that you can refund them...
Luckly my mom is a really smart women.. She called the lady up, and she told her in a Judge Judy tone.. "OK if the box is really empty, send me the entire empty box with all the labels.." the lady freaked out, she was like "ohhh i didn't keep the box, i through it away with its papers..." my mom responded " Thanks for wasting my time, but you cant fool me" and then she hung up the phone... that means the ***** had her item and through the box away.. If she really got a empty box she would have kept the box, so that the mail service can investigate it..After that the scammer never called.... She ran away and tried someone else that she could fool....
In your case the idiot has no proof the box was empty, If he's still insisting to get a refund, do the same thing my mom did... Just tell him your local post office wants to investigate this matter immediately. Tell him to send the entire "empty" package back to You.... with all the labels.. the scammer will leave u alone, and know you are a smart seller... just my 2 cents... GOOD LUCK