They don't offer DHL on the siteFedEx is pretty good and they have fast service but you will pay a premium for it, especially on long overseas journeys such as from the USA. Can't comment on the Aus postage option as I have little experience with them.
Why not try DHL? They have good rates and are reliable plus they do international shipping.
Anything +1k, and declared as such you have to arrange the customs clearance and you will pay duty plus import gst no matter how it ships. In my experience I actually think Aust Post is pretty bloody good these days. I'd reckon if that's the saving that's the line I'd be runningIf the total order value is over $1000, dont use fedex. They will charge you the gst, duty and their "processing fees" as a matter of course. if you go ups or even usps it will normally come through without getting charged the gst.
Also you can use a mail forwarder such as ship2me.com they let you put the declared value on and are pretty efficient and you can consolidate multiple packages. I use them all the time and they are great.
I've used USPS before and it was pretty good. Shipping was about 3 weeks from memory which I was fine with.They don't offer DHL on the site
the next cheapest option is USPS priority international mail at about $65