Back covers are really not used in ebooks. When reduced to the size of an eBook device, the text on the back cover is usually not readable. For this reason, we recommend that the text on the back cover be provided to us to be included inside the book, like a “praise” page, or as blurbs.

Remember:  a backcover on a print book is intended to lure your buyer to your book, to hook them on the story.  When they buy from an online retailer, like Amazon, your prospective reader will have:

•  seen something about the book, somewhere, that interested them;
•  navigated/found their way to your Amazon sales page;
•  Reviewed the description (which you create), 
•  looked at any editorial reviews (like reviews from a newspaper), 
•  looked at any user or reader reviews, 
•  read the look inside, or, 
•  downloaded the free sample, then, 
•  decided to buy the book, 
•  bought the book, 
•  read the book, 
•  got to the end of the book....and then, they'd see the rear cover.

So, in an ebook, by the time a reader saw the back cover, they'd not only have bought the book, but finished it!  For an ebook, that back cover that was created for print is neither needed nor even desirable.