Someone kindly recommended www.bookdepository.co.uk a few weeks ago.  They ship books to Hong Kong and they don't charge for delivery.  Hurrah!

Amazon (both UK and US) also ship to Hong Kong, and offer much bigger discounts.  The problem is the delivery charge - Amazon UK add £4.99 per shipment plus £2.99 per item (i.e. £7.98 for one item, £10.97 for two items), which totally wipes out any savings on books with a cover price of less than about £15.  Amazon US have a lower shipment charge ($4.99), and higher per-item charge ($4.99) but the effect is much the same.

Amazon charge same for delivering one small paperback or a boxed set of hardbacks, which means I will never order a cheap paperback from them, but I may order a more expensive title (this also applies to DVDs, which can make boxed sets excellent value for money).  Maybe they make their charges so high to encourage people to sign up for Amazon Prime (a single annual charge for unlimited shipments), because it's hard to believe that it really costs them £7.98 to ship a paperback to Hong Kong.  

Anyway, Book Depository do things differently.  Order a paperback with a cover price of £7.99 from them and you will likely pay £7.19 including delivery (roughly HK$85), whereas it would cost about HK$150 shipped from Amazon or about HK$120 in a Hong Kong bookstore.

Delivery is by airmail and seems to only take 5-10 days, so no complaints there.  They don't use such elaborate packaging as Amazon, and that must keep costs down, but the books have arrived in good condition.

Unfortunately their customer service isn't so good.  I wanted to order one book in advance of publication, and they were advertising it with a bigger than usual discount, but I decided to wait - and a few days before publication the discount had disappeared completely (whereas Amazon were offering a 60% discount).  I questioned this and got no reply.  I followed up and got a vague response.  I tried again, and this time they suggested I order it from Amazon.  Doh!

Then mysteriously they started offering their usual 10% discount again.  Puzzling.

In fact there are many slightly weird things about their website.  Most books have a very prominent link to Amazon.co.uk that tells you the price and the delivery charge.  I suppose this is for price comparison, and also generates some revenue for them if you do order from Amazon.   They even say:

The Book Depository and Amazon: why do we link to Amazon.co.uk?
We are not in competition with Amazon, we complement Amazon by providing books which have poor availability, offering considerable discounts on certain titles which Amazon are unable to. On the other hand, we recognize that our customers want books quickly and, so, if we do not have stock -- or if Amazon is considerably cheaper -- our customers are able to order direct from Amazon via a link from our website. Our aim is to make "All books available to All", so we make it as easy as possible for you to order and obtain books quickly and efficiently. We hope to give you visibility of other bookseller's availability and prices; you will also find our catalogue on internet marketplaces at Amazon, Play, and other retailers.

Well, I suppose they are not in direct competition with Amazon, and it works for them to co-operate instead of trying to compete.  Probably for sound commercial reasons rather than pure altruism.