But this leaves me with an ethical question. If Amazon had told me "sorry for shiiping you the wrong CD, here's your money back" without asking me to ship the CD back, I would feel free to keep the CD shipped in error and use it as I wished: their error, their solution.
In this case, however, there was no physical product to return. Amazon didn't ask me to delete the MP3s, but they didn't say 'feel free to keep them', either. So my question is: should I delete the mp3s anyway, under the assumption that Amazon isn't paying their wholesaler for the sale? Or should I keep them, under the assumption that Amazon is eating the cost of the error? I'll probably end up doing the former -- I think Amazon would've sent a return lable if this had been a real media situation, so I suspect that's how they're treating it on their end -- but it's a curious artifact of the digital age.