With orders of $25 or more, Amazon offers the AmazonBasics King Microfiber Duvet Cover Set in Bright White for $6.75 with free shipping for Prime members. That's $18 off and a great deal for a king duvet set. It includes a duvet cover and two pillow shams.
Note: This item is offered as part of Amazon's "add-on program" and can only be purchased with an order of $25 or more.
It has very good reviews on over 70 verified purchases at Amazon.