Then send buyer an invoice with increased postage. Click on More Actions > Send Payment Details, far right of item on your My eBay sold page. Change the postage and submit (don't tick "reset" if it's there).
Either print out a label from the RM website or at your post office. You'll need to attach a customs sticker. On that you write what item is, value, weight, sign and date.
Or if they haven't bought/won the item yet and you prefer not to sell to them you can cancel:
