I'm going through this at the moment - or more like ensuring that my parcels do not come via UPS, DHL or Fedex.
If you ship via USPS then the parcels may get opened by Customs/Quarantine if there is no invoice on the outside of the package. They will open the package and inspect the invoice inside. This can delay items for up to a week. But they open them and as long all is ok, they will tape it back up, leave a calling card (an "Australia Post opened your parcel but all was ok" pamphlet) and don't charge you a cent if your parcel is under $1000 AUD value.
Unfortunately the other "courier" companies are not quite so nice.
The reason I choose to ship everything from the US via USPS is because i've heard horror stories of UPS/FEDEX/DHL charging whatever the hell they want for Customs and Quarantine clearances. Do a search on google for UPS customs and you will find stories such as people getting charged $500 customs/quarantine charges for $350 worth of clothes. When quizzed why, it was because they charge for a customs person to enter the details onto a computer and they charged them, wait for it, $50 per line entry to clear the items through customs. Also another story of someone being charged $50 for a $6 glass they brought in from the US. They seem to make values up sometimes also, if something is declared at what they think is too low, they will put their own values on it, regardless of the true value of the item.
It seems to be very random when and if they charge - you seem to have got off lightly for your parcel. Next time you may not be so lucky - avoid UPS/DHL and FEDEX if at all possible for importing from the US. Use USPS as you get auto clearance in most instances if the value of the items is below $1000 (this includes the shipping cost also).
Regarding following them up, save yourself some time and annoyance and don't bother. They are a law unto themselves unfortunately. Save yourself the pain, cop the $33 and move on. Next time don't ship UPS
