I belive it is an ebay glitch across the Australian ebay pages 

The lowest standard Auspost service is $9.70 up to 250gr

Buyers pay on some of my items (individually or combined) up to 250gr for $8.73 (97cents less) 

$8.73 is the ebay discounted label price which offered to sellers if they want to purchase the label from eBay.

The listings and the checkout screens should show buyers $9.70 

I searched on ebay and saw some sellers having the same issue..tons of listings with incorrect $8.73 postage 

I belive the listings with calculated postage having this issue.