Ideally cancel/refund the transactions with buyer's agreement and relist in one listing with one postage for the buyer to buy again. You do that from the dropdown menu for the items on your orders list.
The alternative would be to send them together anyway. Open the unused labels to mark the items as dispatched then add the tracking number to the extra items at View Order Details > Postage > Track Package > dropdown arrow. That has to be done on a desktop browser.
If the buyer paid for the unused labels the cost will be automatically refunded to them in 30 days.
