Tiger_Manc.... go to your post office and ask if they have book post service. It is probably the cheapest way to ship books out that you have bought for Bangladesh. Just make sure that whoever is receiving the books in BD will be nice to their local post office and postman by keeping a nice bokshish ready. And if you can give a corporate/academic address where the books can be delivered, you'll have a higher probability that the books wont go missing.
From my experience in 2007: I took a bus down to NYC with one suitcase full of books (35kg total weight) that I sent as an "M-Bag" which is the USPS book post service, to my mother's Dhaka University department address. The 35 kg of books needed to be boxed in a cardboard carton for shipping, and they chucked that box into a tough plastic sack (the M-bag). I was told that USPS no longer did surface mail, and it would be sent as airmail. This cost me USD220, and 1 month later it arrived in Dhaka. My mother needed to clear a customs declaration form and including bokshish paid the postman Tk.700 total. Well worth it for $6000 of academic hardcover books and papers accumulated over the years.
See this thread