I've been looking at the rates on amazon.co.jp - for books, shipment is 1200￥ (600￥ for CDs/DVDs) with an additional handling charge of 300￥ per item. This makes it cheaper to buy 390￥ manga from eg. the Australian Kino store (if you live in the relevant city) because of the loyalty offers. For anyone else, it probably doesn't make much difference if one buys them in bulk from Japan (which, unfortunately, I don't...yet).
Anyways, if JBP gives the tankoubon a discount of 30%, they'd cost about $8 I think. But for anything above maybe 500￥ in price, it's cheaper to buy in bulk from amazon.co.jp. The cost of artbooks etc would still be way over base price if bought in Australia, so unless they happen to have something I haven't got which I should be getting (eg. the X manga), the JBP booksale wouldn't be worth it for me.
Enough spamming, I should be studying.