As the float on paper checks has decreased, writing checks is no longer a way to extend posting of debit items. (shellfcu.org)
There's a lot more to it, of course, and if you'd like to dig deeper, you can check out Google's extended post on the subject here. (neowin.net)
I'm sure there are plenty of other reasons why an author or publisher might wish to charge more for a book, and I am not going to extend this post unduly by trying to evaluate them. (kindlenationdaily.com)