Why would you use SOD it is not insurance. 

You are only required to use SOD if you are purchasing extra cover for goods over $300.

If you want insurance you need to purchase extra cover.  It is only $1.50 per $100 of coverage.

OP IMO the cheapest and most efficient way to post if you are not an AP business contract is to use eBay satchels and post, (with Click and Send a close second).

Either way you get all the proof you need of sending the goods - if it is lodged over the counter OR if posted in a post box and a single scan is recorded along the way.

Tracking is automatically uploaded saving time as well.

Also saves arguing with disagreeable Post Office staff that need to find new jobs outside of the customer service field.

When I used satchels - I used the lodgement forms too - I just argued with the Post Office staff that I needed them for my business records, as per Australia Tax Office mail register requirement.