how do i purchase a return label from royal mail

[kandhretail]

i have recently had a buyer want to return an item.Unfortunately i could not print or send a label.I communicated this with my buyer and i asked for them to send to me and i would issue a full a full a refund plus the postage.They went on to report the case and ebay has now instructed me to buy a label from royal mail and send it to the customer. i have no clue on how to do that.