Amazon is not the publisher of your book, you are. They are the booksellers
only. If you are in the US (which I presume), I am sure the Writers' Guild has a
contract for that possibility.
My book «Le Maître de Garamond» is on sale on iTunes. They take some
percent, but the rights remain with my original publisher and myself.
For the cover design, you can be as inventive as you will. Have a look on iTunes
or any other, you will see that small or large, the covers are very visible.
Cheers.
Anne Cuneo