The distribution costs are those costs involved in making your book available for purchase through the various wholesale and retail channels. (archwaypublishing.com)
And for what it's worth, my preference would be to make my books available in as many formats as possible. (jimchines.com)
Not making your book available on all the major platforms is cutting off your nose to spite your face and cheating yourself out of profits and readers. (kaitnolan.com)