I have recently just sold via ebay and didn't use the pre paid service. I have posted the item via royal mail special delivery and wanted to update the tracking number and get a refund for the prepaid label I did not use.
If you go to the order details you should be able to add the new tracking number.
If you don't use the Simple Delivery label provided, you or your buyer will be refunded, depending on who paid for postage.
If you selected 'Buyer pays’ when you created the listing, your buyer will automatically be refunded for any Simple Delivery label that isn’t used after 30 days
If you selected ‘Seller pays’ and offered free postage on your listing, you'll need to fill out the Request a refund for Simple Delivery form (link below) for the label that wasn’t used. Make sure to send your request within 14 days of your item selling.