Here is a link to another thread started this week, by a poster with the same problem

http://community.ebay.com.au/t5/Selling/Totally-baffles-Aus-Post-send-notice-saying-I-owe-80-60/m-p/...

Really, if AP is offering an service where people can buy postage online at home, they need to make it a requirement that the person then takes their parcel(s) into their Post Office and get it weighed and checked before it goes in the mail bag/box.

We are already doing their work for them at home, weighing our parcels, computing the postage cost, printing out at our own expense the postage labels.

The PO staff don't always weigh the prepaid parcels I take in, sometimes they do and sometimes they don't. If they don't then they are responsible for letting a underpaid parcel go through (not that any of mine are underpaid).