Well I don't know how much a gamble it is, both the Japanese and UK postal services seem capable of delivering parcels.
However there is an EMS service on offer that allows for compensation if you want, and faster delivery.
(at about an extra 1000 per kilo)
As the charges are by weight and not per consignment it still works out cheaper than Amazon in most cases.
Amazon does hit a point (around 1500 grams? if comparing to EMS shipping) where for larger consignments their flatter rate might make more sense. Myself and 6 classmates split the shipping costs between us once and it was then a more reasonable 800 yen each (although DHL's method of customs declaration left a bit to be desired).
It's contentious ground but the Credit Consumer Act in the UK is meant to apply so if your shipment goes missing you can get your credit card company involved too.
You pays your money and takes your choice as they say.
You can check various postage rates from Japan here
http://www.post.japanpost.jp/cgi-charge ... p?lang=_en
It helpfully explains that a Japanese paperback is about 150 grams and 150 sheets of B4 is 1kg
It also shows that BK1 is only charging at cost and applying the postal services rules as per compensation.
-edit-
it's a pity they don't offer automatic insurance it only costs about 400 yen. Maybe it's too time consuming to be effective. -edit-
There's not an awful lot of choice really.
UK booksellers have an outrageous markup on Japanese imported books and limited selections. So you can either pay high shipping costs (still cheaper than a UK bookstore) or get a friend to repost books from Amazon.jp delivered free in Japan (Can't see that much difference to BK1's costs) or leave your clothes behind and use your luggage allowance to bring home books when visiting Japan or convince someone else to bring them back for you.