Just to add to the correct advice given, as you are asking about refunding postage,  refunds for the paid for, but unused Simple Delivery postage labels go to whoever paid for them 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.

@juliep6069