How would you determine that it makes little difference?  A 500gram satchel or box is currently $7.15 on eBay and $7.20 at the post office.  I pay Australia Post $6.55 GST exempt for the same postage which is around .65 cents less than the $7.15 eBay are offering.  Whilst eBay are offering for a limited time a refund discount and only on certain postage it will not last forever and is for a limited time to act as a sweetener to lure sellers into using their service.  Any seller that has a postage account or can claim the GST component of their postage bill is better off not using eBay labels .  Australia Post charges GST on postage but naturally as eBay does not pay GST sellers cannot claim the GST component of their postage as eBay will not and cannot provide a GST tax invoice for postage fees paid.

Of course small sellers who do not lodge GST returns may benefit a little but my point was to alert the sellers who would usually claim GST back on their postage.