Devious and ******* UPS

UPS delivered a parcel and incorrectly charged us for duties (the duties had already been paid up front). Upon investigation we found they had inserted someone else's waybill and charge sheet into our package, and made us pay for someone else's charges. Email to refund us was first sent 25 March 2019, and plenty follow ups after that. To date, they have not even responded. They are devious and *******.