On Amazon, Advertising Cost of Sales (ACoS) is the inverse of Return on Ad Spend (ROAS). ACoS is calculated by dividing ad spend by ad revenue. To decrease ACoS, ad spend must decrease against fixed ad revenue or ad revenue must increase against fixed ad spend. Ad spend is a function of the number of clicks times the average cost per click. Ad revenue is a function of the number of clicks times the average conversion rate times the average order value. Lowering ACoS comes from either an increase in conversion rate, or a decrease in cost per click. To decrease cost per click, you must either lower your keyword bids or add more longtail, lower-cost keywords. To increase conversion rate, you should optimize your listings by improving product titles, images, and bullets, adding product video and A+ Content, and soliciting product reviews and customer Q&As.