Private sellers have to use Simple Delivery and your items are listed with that service.
As you offer free postage/seller pays, Ebay will make a label (and/or a QR code) available for you to use and charge you for postage.
If you buy your own postage independently of Ebay and don't use the Ebay label, you have to claim the postage cost back from Ebay.
Details here:
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)