I made the change as per your suggestion and it didn't make any difference.
I have done some testing and it seems that it actually comes out more expensive than it would if you had "Packaging" set to "One package per item purchased".
With "Packaging" set to "One package for the entire order" I get the following results based on the same shipping address:
1 Roll $16.65
2 Rolls $37.50
If I use the AusPost online calculator (http://auspost.com.au/apps/postage-calculator.html) I get these results
1 Roll $16.65 (Based on a package dimension of 55cm x 8cm x 8cm and a weight of 1.5kg)
2 Rolls $18.75 (Based on a package dimension of 55cm x 16cm x 8cm and a weight of 3kg)
As you can see there is quite a discrepancy.