Fuel costs, which have stabilized after last year but remain higher - less subsidized - and the relative strength of the dollar. Remember that when you ship overseas, a portion of that postage or freight is paid forward to the delivery country's postal service. It's a tremendously complicated negotiated structure but the weakness of the dollar over the last year or two has tilted the balance of costs a great deal.
As a business shipping bulky, awkward and heavy items, it's on you to investigate the various carriers that service each destination and find the most reasonable option to each. Using just one for convenience is going to mean you have good rates to some places and awful rates elsewhere. I'd suggest (from my experiences) that buyers in Oz and elsewhere are generally accepting of long delivery times to save money. Almost all USPS international stuff goes by air - look at surface shipping and tell your buyers to crack a cold one while they wait.
