It soiunds as though you are purchasing an item from the US, from a seller who has either deliberately or inadvertently opted in to GSP (Global Shipping Program). This is an expensive postage method with some serious drawbacks for buyers (e.g., there is no combining items, but rather each item will have the same high postage); it's not a priority shipping method, even though it's stated on eBay to be "priority"; the company who forward the goods to you after receiving them from the seller will repack (sometimes very badly) and on some occasions will simply not forward the item at all if they decide, according to their own odd ideas, that the item can't be posted to Australia. Drums and guitars and butter knives are among the most infamous examples of such items.
You can contact the seller before purchasing, and ask if they would agree to send to you via another method, but while some sellers will agree, others won't. The reason is that the GSP removes a lot of seller concerns from the whole matter of posting internationally. As far as the seller is concerned, s/he is posting domestically to the warehouse in Erlanger, Kentucky. Seller protection is automatic. From KY, it's Pitney Bowes who do the repacking, the preparing of the Customs declaration, etc.
(Oh, and the separate postage component goes directly to them, not to the seller.)
