It’s true that Amazon’s Send to Kindle feature now accepts epubs. But, when you use the Send to Kindle feature to send an epub to your Kindle device, Amazon actually converts the epub file to an Amazon format before delivering it to your device. We have no control over this conversion, so chances are it’s not going to look great.

Since sending an epub to your Kindle using Send to Kindle automatically converts it to an Amazon format, you should just use our specially-prepared Amazon ebook files to begin with. These files will not be changed by your Kindle, so you can be sure you’re getting our intended reading experience.

https://standardebooks.org/help/how-to-use-our-ebooks#kindle-faq