I recently wrote a small module for Virtuemart (a shopping cart system for Joomla). I couldn't find anything quite like it anywhere else, so I decided to write my own.
Its based around the shipping agreements you can enter into with Australia Post. Australia Post then give you a rate card for shipping your goods to each postcode zone. The rate is calculated on a base package rate, and then a per kilo extra to give you a total price. You can also optionally take out insurance as well.
It works in Virtuemart 1.10 and will most likely work in the more recent versions also. Its fairly simple code and provides a backend configuration page.
You can download the zipped version here
To install it, i normally simply unzip and ftp it to your virtuemart shiping directory (something like: /administrator/components/com_virtuemart/classes/shipping)
The next time you login to your virtuemart admin console, you should be able to go to the shipping section and configure the shipping module, don't forget to enter all your rates!
If you download it, and you really like it, why not buy me a beer?