Usually, if you advertise free postage/seller pays, you can claim back for unused Simple Delivery labels.
However, to make the claim, you have to add a valid tracking number and I am not sure you will be able to use the tracking number for another SD label.
You can only try.
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
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)