Probably by volume. And from repeat customers, owing to the great service, and the great customer service. Once I double ordered a book. (Ordered first copy. Then, about a week later, before it arrived, I ordered it again. No excuse for my carelessness.)

After the second copy arrived, I contacted Fishpond and said that I had mistakenly ordered 2 copies. They promptly responded "Return it, and we'll credit your account."