Í've been chasing bulk (a box of) Signature on Delivery labels and didn't know they existed. I've been "using up" Registered Post labels on parcels because no one has been able to supply me with bulk SOD labels.

You can still use your Registered Post labels on letters and receive a signature and $100 insurance. Technically you're not allowed to use the Registered Post labels on any parcels anymore and you do you don't get the included $100 postage but there may be $50 included at AP's discretion which is not widely publicised. They have said you can still use these labels on parcels until you run out but I've just bought a new box!

The tricky tactic is that my regular PO is adamant that you can only send documents in a letter which I am yet to see proven in writing and until then I'll send whatever fits in a letter as a letter.