The buyer pays eBay for the postage who then send you a link for the label, so you have nothing due to you for postage
Below is taken from the Simple Delivery guidance page,look under Fees & Charges
https://www.ebay.co.uk/help/selling/posting-items/setting-postage-options/simple-delivery?id=5575
Unless you've offered free postage, your buyer is charged for the delivery cost at checkout and the amount is then paid to eBay for the label. If you decide not to use the prepaid label, you'll need to purchase your own label instead at your own cost.