You could try here:
オンライン書店ビーケーワン
I can't remember the shipping costs compared to Amazon but if I remember correctly it was cheaper.
Although that may depend on country.
Come to think of it I think I may have used 船便, which is obviously going to be really cheap, but takes between 1 - 3 months to get to you.
I suppose it all depends on how patient you are.
Well, here is the 海外へのお届け section of the FAQ anyway:
オンライン書店ビーケーワン： ガイド＆ヘルプ＞ご利用ガイド