Not related to HarperCollins, but by the same token, as much as I like using Amazon, I'll always be checking the prices at BooksOnBoard too before I buy.
I'd been wanting to buy the book about German goalkeeper Robert Enke (A life Too Short) for a while but was waiting to see if it'd come down to nearer £5. Kobo, Sony, Waterstones, Google - all £6.49. I nearly bought it from Amazon for £5.64, but I'd remembered someone on here recommending BooksOnBoard so thought I'd check there first.... bought it from them for £4.35.