One solution, but with the buyer's acceptance,  is to cancel the order citing Buyer Requests to Cancel, this fully refunds the buyer,  and your seller performance is not affected.

You can then relist the items,  together on one listing,  therefore giving you one postage label to buy and that all important tracking number here covering both items , for the buyer to buy and pay again.

A faff I know,  and not ideal I know,  but you did ask for a solution to present itself !  This is the only one where you as a seller,  are protected with tracking for both items.

Unused labels are refunded to whoever paid for the labels this way:

• If you selected 'Buyer pays’ when you created the listing, your buyer will automatically be refunded for any Simple Delivery label that isn’t used after 30 days

• If you selected ‘Seller pays’ and offered free postage on your listing, you'll need to fill out the Request a refund for Simple Delivery form for the label that wasn’t used. Make sure to send your request within 14 days of your item selling.  

@chrisd0407 

@theretrodetective