I’m guessing it’s not a “tax”, despite what they told you, but a fee imposed by the restaurant. Somerville has had a bylaw for a while (like a decade or more) banning polystyrene/styrofoam in takeout containers and restaurants. They’ve also since banned plastic straw and stirrers, as well as plastic bags. I don’t know if they’ve fully banned plastic containers yet, but it wouldn’t surprise me if it’s coming soon. I suspect the restaurant is charging you a “tax” because the other materials for their takeout containers are more expensive.