Or you can keep it simple and simply tell Calibre to convert the Amazon book to epub. It rarely takes more than about 30 seconds for an average size book. I do it all the time and it's trivial and fast and has never failed yet. I have no doubt the result contains unoptimized portions that purists and any watching angels will frown on but the books always work fine for readers like me.
Most of the books I've read this year were purchased at Amazon, downloaded to my PC, quickly converted in calibre. Then, when I read I download it to my Kindle and put the epub on my phone in Moon+. I also keep the Amazon version in my phone's Kindle to make syncing easier.
The only time I've ever had to spend more time converting is when the TOC is inadequate. A few books have TOC entries for sections of the book but not for chapters and I use Calibre to add a full TOC to the epub, convert that to mobi and email it through Amazon to my Kindle and read that instead of the original Amazon version. I like a full table of contents.
I've sometimes wondered if that'll get me in trouble with Amazon but of course they know I bought the book from them so I guess they don't care. At one point I did call their support and ask about this and the guy I talked with said he can't tell me that it's okay to do that but he's never heard anyone discuss that as a problem either. He didn't tell me he does it but he knew exactly what and how I was doing it and he gave me the feeling he did the same thing.
Barry