I've purchased from the US in the past both via ebay & online stores. I think the general consensus is to use USPS and no UPS (I've never used UPS but it's very unreliable from what I hear and you're alot more likely to get slugged with extra duties and fees of sorts).
From what I know as long as your purchase is under AU $500 then there isn't any extra taxes or costs. I've never been into the over $500 domain.
My US Ebay purchase was a heart rate monitor, in that case the seller did declare it as a gift and I didn't even ask - maybe it saved him extra costs, don't know.
I've purchased stuff off both DropNZone & Speedgoat (both online bike shops) and they declared the stuff as bike parts which they were including their true value (the price I paid for them), postage was around the $30US mark from Speedgoat and generally it depends on the size and weight of your gear, generally it's more cost effective to buy more than one item at a time.
Be pretty careful though scams seem to be alot more rampant with ebay sellers targeting US buyers it seems particularly with bikes / bike parts. If it sounds too good to be true then it probably is, and be very careful if they try to settle outside of ebay & second chance type offers.
If you have the time you will find alot of info about buying from OS in the thread below:
http://www.mtbdirt.com.au/modules.php?name=Forums&file=viewtopic&t=4629