Version note: Magento 1.7, 2.1
The amounts you specify for products, discounts, etc. on your Skystore is always in Inc Tax. This means that if you set up a product as $55, the Skystore will calculate that the product price is made up of $50 (product) + $5 (tax, based off Australian sites with a 10% tax rate).
When a customer checks out, whether they are charged tax or not will depend on the shipping address that they have chosen to ship the order to. If not the tax value will be deducted from the values, and this applies to all values including shipping.
This means that if you have specified $20 shipping for international orders, the customer will actually only be charged $18.18 because the Skystore will remove the tax portion, as Tax is not applicable on International orders.
In order to prevent this from occurring, if you have specific rates for international orders you need to enter the values in as what you want to charge plus your tax multiplier.
For example, Australian tax is 10%, so if you wanted to charge $20 on international shipping, you would actually need to calculate $20 + 10% and set that as the freight price (in this example, that would be $22.00).
Then when the customer gets to the shipping section of checkout, it will determine that they require the international shipping option, which is $22, then it will take of the tax of $2, leaving the customer with a shipping cost of $20.