There's a box on the Straz site that's checked, and if you don't remove the check mark you automatically donate $5 per ticket, on top of the regular ticket price and service charge.
The money goes for arts education at the Straz, which is of course a great thing to donate money to.
There's apparently no intent to deceive. Paul Bilyeu, the director of public relations for the Straz, pointed out that the box is so prominent that "it really couldn't be more obvious." But we all overlook obvious things all the time when we're multitasking or are otherwise distracted, and no doubt many of us have unwittingly cost ourselves $5 by not noticing that box.