Hi,
The Australia Post plugin only builds the name of the method ("AustPost" + the service name returned by the API), it never adds the price. So the second amount comes from the display of the shipping methods in the checkout, and there is one usual cause:
The price display setting. If it is set to display the prices both with and without tax, you get two amounts for each method, and when the shipping has no tax the two amounts are identical. You can change that in the options of the "shipping" block of your checkout workflow (HikaShop > System > Configuration > Checkout tab, click on the shipping block), or in HikaShop > System > Configuration > Main tab > Price display.