Error Message: The Unit value must be at least 1

Australia Post won't validate shipments if a product has a $0.00 value for international orders sent via ParcelSend/eParcel.

You'll need to assign the product(s) with a value of $1.00 under the unit price (this is the "minimum" cost that AusPost will accept), you will then be able to save the shipment and continue per normal.