According to the Desiring God blog, ithey are about to have a massive sale! Every book in the Desiring God Ministries online store (i.e. John Piper and others) is on sale for only $5 (at current rates, about £2.50!) on Wednesday 27th and Thursday 28th this week.
I presume it is still okay for international customers, so give it a try and see what happens. I am certainly going to be there adding a couple of books to my collection.