Combining postage has been an issue with SD from day one.
Were these were 2 separate purchases, then purchase the second label, it automatically marks the item as dispatched, below I will explain how the refund for the unused item is made.
Be aware, both will then be marked as dispatched, but you will only have proof of one tracking, therefore it would be possible for the buyer to open a case for item not received for one item.
Although a faff , best way forward would be to cancel both orders, with the buyer's permission so it doesn't affect your seller performance this also refunds them. Then, relist both together as one, so there' s that one important tracking number covering both items.
For unused SD labels:
- 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. Also ensure you add the replacement tracking number of the service used, and it shows delivery.
@mrvenal