From Cal's website:

Ordering information:

The British Bore Rifle.........................$90
The .600 Nitro Express......................$35 when ordered with the BBR (full price is $70)

Shipping (pick one)
1) to US addresses:
one book media mail........................$8
one or two books priority mail.........$15
three or four books priority mail......$22
2) international addresses:
one or two books priority mail.........$52
three or four books priority mail......$67

So I'm guessing $52-$67/4=$35 per book is the potential international postage saving, less the cost of Aust postage, probably around A$12 for an express bag.

I've sent off an email to Cal to see if we can do this.