Contact the police in his area, file a stolen item report. Have the person who was supposed to receive the item do the same. You can still attempt to file a loss claim with the USPS, I have seen them deliver packages to the wrong house and update their systems to reflect the item as delivered.
Notify the person and let them know you are filing a report for stolen property that is in USPS care at your request until delivered to intended recipient...
My money is that he/she has you RC car and is committing fraud.