The Book Depository pretty much always works out cheaper than Amazon, unless it's a very expensive book and Amazon have a third off it or something. I live abroad and order from the Book Depository all the time - I only use Amazon if I really need something to get here fast, because Amazon is a little bit faster but the postage costs are huge.
I live in Germany and order from amazon UK and amazon Germany. What I usually do is check both in terms of prices and postage - you'll have to do some quick pound/Euro conversions. You will have to put the address into their system as if you are ordering to get the postage price. But it's unlikely that your friend will have to pay any duties.
Spain is in Europe so there are no duties to pay. I would decide what book you want and then have a look on Amazon uk and see how muchpostage will be. often there is free postage even if you send abroad. If you have a Uk amazon account then you can log in any Amazon site with your original details and pay as id paying in the uk.
I wasn't sure which forum to put this in. I wanted to send a book as a present to a friend in Spain. But I wondered how to do that. Could I buy something off amazon-uk for example or would my friend have to pay some kinds of excise duties on arrival. Also I presume it would go from a UK depot and be expensive in shipping charges. Or could I order something from amazon-spain - would that be possible with a UK debit card? I presume shipping charges would be lower if the book was in a spanish depot. Would there be any other charges me or my friend would have to pay then?