Amazon has Solid Duvet Cover Set (Twin,Full/Queen,King) in 6 colors on sale for only $9.99-$14.99 with promo code TUEP3SUB. (50% off) Shipping is free with prime or order $25+
After code TUEP3SUB price:
Twin size for $9.99
https://www.amazon.com/dp/B071DCCY7B/
Full/Queen size for $13.49
https://www.amazon.com/dp/B071J17YZJ/
King size for $14.99
https://www.amazon.com/dp/B071LLXRCL/