Do you mean the buyer requested cancellation after it was shipped? If so they can't "cancel" and will have to wait until they receive item and then request a refund at which point you can send them a return label. At this point (after paid for and shipped) it would have to be a refund not a cancellation.
Or do you mean eBay themselves cancelled? If so what was reason given, was it an item that wasn't allowed to be sold/against policy? Either way once paid for and shipped out it would no longer be a "cancellation" it would be refund I would think but as mentioned above that can't be done until item was received and sent back. Can you give any more details?
