When I attend opera, I mostly see gray and white hair in the audience. I am not sure that people of that age wear T-shirts much, if at all, another reason the purchase of one while buying tickets seems iffy.

Then there’s the issue of combining types of experiences. When I call to buy a ticket, I am thinking seating and calendar and parking, not consumer goods. I suppose if you offered a discount for buying the shirt combined with the ticket, you might increase shirt sales. Opera being what it is, you probably make a profit on the shirts, but not the tickets.