Liz,
http://www.auspost.com.au/BCP/0,1080,CH213...57EMO19,00.html
CoD is essentially set up for businesses which dont have an account with someone and cbf setting one up to send one invoice - so they do it via CoD.
I dont really know what the ramifications of not picking it up might be - call someone at Aus. Post and ask them. Presumably if its not signed for and not delivered and not paid for the sale doesnt happen and the onus is on the sender. Which is why some companies dont like CoD.
As far as I can figure, the point of sale is when you pay Aus Post for the item. up until then there is not formal contract to buy established and no exchange of goods has taken place.