Yeah there isn't anything that can be done if FedEx shows the boxes delivered and the customer says they were not delivered. Someone isn't telling the truth and it is impossible to figure out who it is. I've run into a few times over the years. On average the customers are more likely to try and steal stuff than the freight company delivery guy, but there are cases where it is the other way around.

I've even had Moparts members tell me they didn't get stuff that I shipped although the carrier showed it delivered. One time the item went to the wrong house and the guy found it. Another time the guy stopped complaining when I showed him the delivery confirmation. One time I replaced the parts since I trusted the guy. Stuff happens and it is impossible to know what happened without a 24 hour video feed of their front door.