A to Z case upheld by amazon despite purchasing the label through Amazon and Royal Mail tracking shows that the order was delivered on time

In short a customer ordered an item, I purchased the postage label through Amazon. The Item was dispatched on time. When I input the tracking number from the label on the Royal mail Track and trace website it shows that the package was delivered on time. I appealed the A to Z case and attached a screenshot of the delivery confirmation from the royal mail website and amazon have upheld the claim and charged my account for the refund. This must be tantamount to theft as every detail shows that the package was dispatched and delivered on time...Any help in taking this further would be much appreciated