If you google interparcel, they are a website which compares prices for courier services and then books the service for you. You can pay online with paypal if you like which I find more convenient and then you just have to package your parcel and print the shipping labels off. I have never had any problem with them and I have never paid more than it would cost me to send via Australia Post.

However all of my parcels are about 2kg or more so anything under 2kg may not be cheaper than prepaid parcel bags from Australia Post.