Retailers generated nearly $31 million in online coupon sales through RetailMeNot.com in May, according to the site. That's over a 42% increase compared to the same time last year, and that's just through RetailMeNot. The company has released a report looking at coupon usage and search behavior in the month of May. RetailMeNot says it's in a unique position to track trends in the coupon industry, as it is the most highly trafficked coupon site and the only site with over 450,000 coupons for both online and in-store offers.