Some publishing platform offer that they extend your epub with a „by the author“ page and add the links for the store the book is sold in. You have no control how it looks like, which books are advertised and it only works with epub2. Draft2Digital threatens with „nice EPUB3 you have there, what a pitty we can not change anything inside“
The whole selfpublishing industry really has the worst tools. Features by platforms just do not work with a now 14 year old file format. >>