Unfortunately, if you want tracking via Aus Post, standard (or express) parcel services are the only way to get it. 
Large letters are the cheapest way to post for small items, but you can't register a large letter, nor add signature on delivery, but you can add insurance (the registered pre-paids are for documents only). If your package is less than 50g, then postage to most countries will be $2.95 for a large letter. Insurance for the first $100 is $9.60 (which is disgusting, I can insure US$1k for the same amount via other postal services 
), so total would be $12.55 for an insured but non-tracked large letter, however buying insurance usually results in the package being traceable. If your package is 51g - 250g, then postage would be $8.00, bringing the total to $17.60 (it's hard to get them under 50g, because you can only add insurance to large letters if you use the newer CN23 customs declaration form, which is twice the size of the older green CN22 forms, forcing people to use larger than necessary packaging in a lot of cases, and weights about 20g itself - note the costs for a larger letter to NZ and Asia are lower, $1.95 for under 50g, $5~something for 51-250g).
Personally, I generally stick with the small, large letters, and set up my postage costs in a way that I can upgrade to a standard parcel with tracking if they've bought a certain amount of items. My items are fairly low value, so I'm not risking massive amounts of money (not on a single package, anyway), but it is a risk (and to be honest, not one I'm prepared to take on eBay, as I only sell internationally elsewhere). I do have some higher value items, so for those I charge for the higher parcel costs, but I include the cost of registered post in the pricing for Australian buyers, and charge the difference for international buyers. eg if an item would be $70, it will be listed around $77, no extra post for Aussies, and $15 shipping for international buyers, which doesn't come across so bad, then.
If you have an ABN, you may want to consider signing up for Aus Post's Business 250 discount scheme, as if you qualify (by sending 250+ parcels per year, whcih includes domestic and international), you will get a 10% discount on standard and express parcel service to international destinations (I pay $21.35 for packages to the US, still horribly high, I know).
There are some alternatives if your volume is high enough - I'm starting to look at DHL. There's also Skippy Post, but there's some drawbacks with them as while cheaper, you have to ship a box of packages to them, then they send everything over to Singapore and ship the individual packages from there, where they'll get a tracking number, so it's a fair bit slower and creates a few issues with eBay's late shipment stuff, and likely their ETAs as well.
