If you keep getting following error on osCommerce Checkout > Shipping Method page (for any or all post methods on the AusPOST module parcel, express):
ERROR: We apologise, we are unable to obtain a valid quote from the Australia Post Server.
If you would like to use this option as your shipping method, please contact us.
You first need to first check that you have added weight for your product. Also check that you have provided Dispatch Postcode in the Admin > Modules > Shipping > AusPOST Parcel Post.
Also go to AusPOST eDeliver website and get registered. http://www.edeliver.com.au/Templates/ifs/IFS_DRC_Terms.htm.
For me, it was missing product weight that caused the error.