So far as I see GST issue is confined - in my case - to sales made last night.  Invoices I sent for sales made today were fine; but when I re-sent today the invoice from last night the buyer said " Unfortunately it is still adding GST & showing a Total $138.60 at checkout. Another time it shows a $100 + Postage of $48 & with a Total of $126??? but when I go to Pay it reverts back to charging GST with a Total to Pay $138.60" (Total should be $100 +$ 26 postage). I will try re-sending invoice one more time tomorrow - if the problem still occurs I will just ask them to pay and will refund the $12 GST. If it is just a one-off glitch I feel it is hardly worth my struggling with an eBay chat.