How Do We Get A Refund On A Letter Never Received Overseas And Insured?

The post office in Cleveland Texas keeps giving us the run around on how to get the refund on the insured letter and told us to call the post master and called her all she said is call main post office because they can't do anything about it. The back of the postage and insured paper say that we should get refunded 75% of insured since it was sent over seas, but the post office keeps saying they can't do anything about it.