If you are referring to a simple delivery label, for which payment is taken immediately when your item is sold, then usually you do not have to do anything when an order is cancelled. In my case I paid for the label and when a buyer requested a cancellation and I accepted, I was refunded the postage (automatically) 2-3 days later. This has been the situation with previous cancellations too. You just have to monitor your 'payments', there is no notification of any kind.