My best guess is that you are using calculated postage in your listings instead of flat rate postage?

If you were using flat rate postage,  i.e. the Aust Post satchels - extra small, small, large, etc, you would know the exact price to charge.    The below sample is what ebay will charge you for using their labels, you just add a small amount for the purchase of satchels, like 25c

 EBAY POSTAGE AS AT 15 NOVEMBER 2025