What they are doing is charging 10% GST for the item, plus you are paying the 'ebay global shippings handling fee' so then they gotta charge another 10% on their 'service'. This is how its adding up to 21% or more and they group it all together and call it 'import fees'. if you buy from a seller not shipping by ebay global shipping, you will just be paying 10%
so to make it clearer through ebay global you pay....
item + shipping to ebay global shipping center + 10% on this
plus
shipping from ebay global shipping center to Australia+ i assume they are adding in their 'handling service fee' in this as well, + 10% on all this aswell
so all added up it comes to more than just 10% on the item and shipping, but you are paying 2 lots of 10% basically because there is 2 transactions. The increase if any, is just ebay putting up their handling/shipping/import fees from the global shipping center to Australia.