hquek:Hi marklim,
So far I've only tried dangdang and peeka for china, and vpost for Europe. Personally, vpost was very expensive option for me, and I think the china option also ex.
For peeka, if you have the exact dimensions, it'll be easier to gauge the final bill. Choose economy and you won't have to worry how heavy these are. This service is fast and transparent (status update available). But in my case, though I tried to order only books of the same size, I paid more than what I would have under dangdang.
My favorite probably is dangdang becos I know exactly how much I need to pay. Sometimes the amount on the stamp is higher, other times lower than what I paid. Only problem is that it can be slower than putting it on a snail and having the snail crawl all the way here. But my books have so far arrived safely. My preference also becos I got too many books and I can wait. If you can't, then better to choose another option.
Thank you for sharing hquek

I guess for heavy and bulky volumes, dangdang is the only viable option since it doesn't go by weight.
I am surprised peeka cost more for you. Was your package size considered large (in peeka's lingo), i.e. between 60,001 - 200,000 cubic cm? I am assuming that for single-volume books (say around 400 pages), it would take many of them to reach that range. So if i buy less than 10 books, my package size should be medium and peeka's shipping rate for medium is quite low indeed.
Also, for peeka, did you have to pay GST and handling fee on top of shipping fee? Are these percentages out of shipping cost or (shipping cost+product cost) or something else?
In any case, great to hear your books arrive safely! At least the tradeoff is between speed and cost and not security.