The people over at Amazon.com are swift.
As I had discovered earlier, ordering and downloading an e-document on Amazon is a breeze, even though I received something different from what was offered.
Well, they have sent a quick and polite promise of refund:
I've requested a refund of $6.00 to your credit card. This refund should go through within 2-3 business days and will appear as a credit on your next credit card billing statement.
(Thank you, folks!)
Have they fixed the error that led to this situation in the first place?
Not yet. As I write, the digtial edition of the book at $ 6 is still on offer: http://www.amazon.com/gp/product/0684841479