Under the "International Postage" section when editing an item:

1. Select "Flat rate cost to all buyers".

2. Under "Post to" select "Custom Location"

3. Tick the "United States" checkbox.

4. Enter a value of $14.10 or higher if it's a heavier item.

Save the changes.

Use the Australia Post postage calculator to determine the cost.

Always add in 50 cents to a dollar to make sure you're covered when eBay takes their cut out of the postage amount., and if you charge for packaging supplies and handling.