There are other ebook-sellers, you know, like
http://www.booksamillion.com/ebooks. They just use simple epub.
A rather esoteric tip: if you know German or rather know someone in Germany, you can buy English language ebooks often at a much cheaper price by buying them at
https://www.ebook.de/. (Germany has fixed prices for printed books, but not for ebooks; whereas it seems as if it's the other way around in the US. They also allow sales for their ebooks, whereas in the US ebooks always seem to be excluded for sales promotions.) They also just use simple epub. I never had a problem converting any.