- You cannot add that amount of "Ergo21 LiquiCell Coccyx Seat Cushion for Wheelchair, Office Chair, Car, Tailbone Pain Relief, Lower Back & Sciatica Buttock Pain, Coccyx Pillow for Sitting (20" W x 18" L x 2.5" H) (022228885290)" to the cart because there is not enough stock (0 remaining).