Seems like it would be better to incentivize people to use their own mugs. At my local coffee shop if you use your own mug when you purchase a drink they give

Seems like it would be better to incentivize people to use their own mugs. At my local coffee shop if you use your own mug when you purchase a drink they give you a little wooden 'nickel'. They have a series of jars on the counter, each representing a local charity. You drop your wooden nickel into the jar of your choice and they donate a nickel to that charity.