Polyal, I've been importing vehicle parts for many years now and as long as the total combined cost of the consignment isn't greater than $1000, the import won't attract any duty or tax. That's the value of the items, shipping costs are not part of the value. I'm fairly certain the self assessed clearance (SAC) that is mentioned in the fact sheet is the docket that's filled out by the seller who places the value of the items on it and attaches it to the parcel/package/etc...
Not that I could locate it, perhaps the particular restriction may have changed, however I thought that the $1000 threshold was over a 12 month period. It doesn't seem to be jumping out at me on the customs website, so perhaps this restriction may have been removed because of the GFC??? to promote the global economy.
http://www.customs.gov.au/site/page5549.asp#DutyOf course, to be completely confident, its just a quick email enquiry to Customs to be certain about any tax or duty that may be imposed.