I think you were just unlucky with the customer service rep who happened to take your call.  If you'd called back again, odds are you would have been given a different answer.  You have up to 30 days to cancel a label.  AFAIK I think it's even possible after that but they have to refund you with a money order rather than through the method you paid with.  You can email  clickandsend@auspost.com.au with the details (this can be slow as they often have a huge backlog) or you can contact them on 1300 662 710.