When your customer purchases an EPUB file, they can follow these steps:
Go here: https://www.amazon.com/sendtokindle.
Sign in to your Amazon account.
Upload your file.
Open your Kindle or Kindle app — your book will appear in your library within a few minutes.
*Make sure you're logged into the same Amazon account your Kindle uses. If you don’t see your book, tap “Sync” in your Kindle app