"I can understand your frustration with having to wait longer to get our Baggage Service Guarantee voucher. We use these vouchers for tracking purposes, and understand it takes additional time to receive the voucher. However, we do need these codes entered into our system so we can try to improve our service. In the future, we will need you to get the voucher and enter it into our system if you would like to receive additional miles."

how about using the Customer Service case number on the email acknowledgment?