From their site:
Find the book you're looking for from our database of over 1.2 million titles, all discounted by 10%. And take advantage of our special offer of free shipping* to customers paying with PayPal for the period of this promotion.
I know it says "Offer Available between 16/7/2008 and 30/7/2008" at the moment, but according to PayPal offers, the promotion is now between 24/09/2008 and 01/10/2008.