I had a similar but opposite problem.
I have been out of the system for over 13 years now and the system may have changed.
If I sent mail to an FOI in Brisbane I had to send it to a PO Box in Canberra. - go figure, the then BN CASA office was just across the river from me but I could not hand deliver.
Therefore I would use Express mail to get tracking and delivery notices on the packages.
However, one package went missing. Never arrived into the CASA mail? system - that was what CASA said.
AustPost confirmed that it had been delivered. CASA in Canberra could/would not acknowledge that the package had been delivered as the Express mail package did not require a signature.
CASA would only acknowledge receipt of postal packages that required a signature on delivery.
Result was more cost for the Client and more money for AustPost.