That would suggest you actually purchased a postage label from eBay, which is something you can print out and attach to the package, then send (i.e. you pay for the postage via eBay, as the charge gets added to your account), and then the barcode / QR code from that label is scanned when you lodge it.

You can access this option from the dropdown menu next to any order and select "print postage label", but you would need to print it out and attach it to the package to make use of it.