It's a common upsell at many places. It costs them pennies, but they will charge anywhere from $20-50 if you want it. I've even seen it lumped into $500-750 dealer add on packages when you buy a new car. As GoingMyWay mentioned, Costco just lumps it into their standard installation fees rather than milking you (well, they may have done away with the installation fees now and built everything into the cost per tire).