Just to add to PJ's post, the GSP (or Global Shipping Program) is something run by a company called Pitney Bowes, and what basically happens is that the seller themselves do not ship overseas directly to the buyer, but send the package to a facility where Pitney Bowes process (and typically repackage) the item before sending them on.
In those cases, these high postage charges are decided upon by Pitney Bowes, and the seller either receives $0 for P&H, or I would suppose whatever the price of their domestic rate is (eg say they charge $5 to ship an item to another US buyer, the seller receives $5, but Pitney Bowes add their own amount on to that, and that is the final charge you see on eBay when you're looking at an item. If the item is very expensive, then import charges can also be added on to the final charge so that they're not payable when the item arrives in Oz).
I agree the amounts can be outrageous for small items. I buy quite a lot of craft supplies, and the packages can be very small and light weight. Before the introduction of the GSP, I could regularly pay around $4 postage for the items and get additional items for free or very little extra, but if the seller is using the GSP, not only is postage suddenlly $30 or more, but combine postage isn't an option, so if I want another similar item, the charge could be $65 rather than $5.
Many sellers find this program attractive, as it reduces the risk (to them) involved in sending items overseas, but it's not very cost-effective for the buyer of smaller items. Other sellers don't actually know they've been opted into this program, so sometimes you might find that the seller either never meant to make overseas shipping available, or that they'll agree to bypass the GSP and send the item to you directly using normal postage methods (better to ask before buying, though).
