If the airline can not secure enough volunteers the first go around, they will often sweeten the deal with additional cash and other benefits like drink coupons, upgrades, and more. (travelinsurancereview.net)
For instance, I receive on checking in a courtesy drink coupon. (accorhotels.com)
If you do, you will also get a display stand, six free drink coupons, and a tote bag. (perfectly-nintendo.com)