Under Simple delivery, Free Postage means eBay charge you for the provided tracked label. If you do not use it, you need to manually cancel the label for a refund.
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.
Be aware, adding just stamps to an item, you'll have no tracking to prove delivery, so your payouts will be held for a minimum of 14 days before release.
@freckoid