just take it back and send him cash. i have sent hundreds of dollars and never had an issue. thats what i would do, in fact , i just sent 50 in the mail.
Consider yourself lucky. In my 18 or so years with the PO I know of at least 2 dozen folks who were terminated for stealing things out of the mail......One of them trained my wife on her first day at the PO.....she witnessed him stealing......