Amazon has 11-Count Swiffer 360° Heavy Duty Duster Refills on sale for $7.79 when you ‘clip’ the $3.58 coupon on the product page and checkout via subscribe & save. Shipping is free w/ Prime or on orders $25+.
This price is $2.20 lower ( 22% savings) than the next best price from a reputable merchant with prices starting from $9.99 at the time of this posting.