If you read through this post I mention the challenges of using the dimensions from the items (only becomes a problem with multiple items).
I realise the hard coded option isn't the best method but calculating the total package size if the package contains multiple items is extremely challenging. For a solution in this thread I have suggested that you push the hardcoded size up if you are having problems with underquoting. The dimensions that are in the module were set where they are (100x100x100) because of the item size a client had in his store.
The method I will eventually code in (and that is at the top of the list) is getting the cube root of the total shopping cart volume (multiple items)... so adding up all their volumes then sending the cube root to the module. This also has some issues with parcels being physically too big for Australia Post getting through (extremely long items) but it should give a much more accurate postage calculation.
So the logic I plan to implement is
If 1 item, send item dimensions to Australia Post
If more than 1 item, calculate total volume of all items in cart and forward the cube root of that volume to the Australia Post module.
If you are interested in why multiple items are challenging, take a look at the following site, it has an excellent write up:
http://www.plambeck.org/oldhtml/mathema ... /index.htm
As for an ETA on the size calculation, I was planning to spend some time on it tomorrow. Registered post I will look into, can honestly say it is something I have completely overlooked until you pointed it out.