I don't think you can add postage these days. You can check if the option to send an invoice exists via the dropdown menu for the item on your orders page. If not the best thing to do is to cancel the current transaction (also via the dropdown menu) and relist with postage on the listing for the buyer to buy again. If they've paid a refund will be issued in the process of the cancellation.
If you'd rather not send the item you can just cancel and relist as collection again. And put their ID on your blocked bidder list:
https://www.ebay.co.uk/bmgt/BuyerBlock
