My customer has been charged £21.54 plus £5.81 import charges for me to send ONE postcard to Australia through ebays Simple Delivery. How can I get this revoked. It is an absolute rip off and a disgrace that a company would stoop so low to make a profit.

You have to opt out of eBay's Global Shipping here: https://www.ebay.co.uk/ship/prf and then when you relist you will be able to add your own overseas postage.