Car parts are most likely to be duty free.
transponder and speedies may have duty payable.
The postage is added to value of the goods before duty is calculated.
For example
value of goods + postage + duty = total
duty amounts vary from item to item.
GST is 10% of total.
for a $850 , $50 postage, 5% duty ($42.50) = $942.50 GST = 94.25
amount payable would be for duty $42.50 and GST $94.25. total $136.75.

If the duty and GST is under $50au, customs usuall don't care

This is only an example, figures are an estimate only. all currencies are in $AU.