If they've paid for item + postage you can buy your own postage and add the tracking and mark dispatched manually via the dropdown actions menu for the item on your orders page.
If you have any problems with that it might be simpler to cancel this transaction (with buyer's agreement - a refund will be issued in the process) and relist with postage on the listing for them to buy and pay again. Click on the dropdown menu arrow again and select Cancel order > Buyer asked to cancel.
@smiler_03