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 label to mark the item as dispatched then add the tracking number to the extra item at View Order Details > Postage > Track Package > dropdown arrow. That has to be done on a desktop browser but some people have problems doing this. If you can't manage it the payment will be held for 14 days.
If the buyer paid for the unused label the cost will be automatically refunded to them in 30 days.