The best option will probably depend on you volume of overseas shipments, and average value of the orders.
The cheapest Australia Post option for a tracked shipment to the US is Pack & Track (registered is not officially trackable in Aus, and USPS stopped showing the results on the website some months back).
That's $21.10 for a package up to 500g ($19.00 if you're a Business 250 member) - that service is more expensive to other countries, eg $25~odd to the UK / Europe, and about $18 to New Zealand (again, less if you qualify for B250 - you need an ABN to apply, and keep in mind you'll no longer get the option to add extra cover to a Pack & Track parcel, but you can add it to international express services, which you also get a discount on, in case you ever use that).
I find my Pack and Track shipments average 2-3 weeks for delivery to the US.
An alternative option is Skippy Post. You ship your packages to them, and then they ship out via Singapore Post, so it can be a lot cheaper (especially if you send several packages at once, because you have to pay $11.00 per box when you ship your stuff to them, so it's more cost effective the more packages you send at once, plus the basic shippng rates are discounted after 3 [or 4?] packages in the same shipment).
I'm not too clued in on the general rates for Skippy, but I remember looking at them for a package to somewhere that pack & track wasn't available and it was about $10-$15 less than Aus Post (before counting the box fee and extra cover).
The main drawback for Skippy is that transit time is quoted as 20-30 days, so you'll probably still find you'll get "where's my item" Q's from a lot of people. (From my own experience, a lot of US buyers start thinking there's a problem as soon as it hits 2 weeks, but Skippy is trackable so that might allay some of them).
