Buyers have 30 minutes to cancel their own orders, but you can cancel orders up until the minute that you ship. If you have not yet shipped and confirmed the order, you can cancel it in Manage Orders or by using an Order Cancellation feed.
When you cancel an order, Amazon automatically updates the order status in the buyer's Amazon account and sends an email notification to the buyer.
If you have already shipped and confirmed the order, you will need to let the buyer know the order is already on its way. Request that they return the package to you when they receive it. Upon receipt of the return, you should issue a refund for the order.
See also Cancel an order or multiple orders.
An order can only be canceled if the shipment has not yet been confirmed. If the buyer still wants the product, ask them to place a new order.
Orders are listed as "Pending" while Amazon is verifying the payment method provided by the buyer. Pending orders do not include the buyer's shipping address or contact information and should not be shipped, even if the buyer contacts you directly. If buyers contact you while their order is in "Pending" status, refer them to Amazon Customer Service.
Once the payment method has been verified, the Confirm shipment and Cancel order buttons will become available in Manage Orders. In addition, the order will appear in both the Orders Report and the Unshipped Orders Report. At this point, the order should be shipped (or canceled, if necessary).