https://www.ebay.co.uk/sellercentre/postage/simple-delivery#eligible
If you don’t have a printer, just use the QR code from your My eBay and Seller Hub Order Details page, or the one that was emailed to you. There’s no need to print out the postage label, just take the QR code with you when you drop off the item at the courier location.
If you’re posting with Royal Mail and book a collection, the label will be brought to you when your item is being collected.