Dunkin Donuts Charged more than what it should be for a breakfast sandwich Cambridge Massachusetts

Bagel+Sausage+Cheese Sandwich, #7 on the menu is $4.99 + 5% MA tax = $5.25, but DD charged $5.45. On showing the math, the manager said that he was going to stick with what system said as it came from the corporate and whether I wanted the sandwich or not. Finally, on raising my voice a bit, he charged me the right amount.

This is not the first time it has happened. I have reported the same thing in the last two times I went there and despite my complains, DD or the manager did not take any steps to correct it.

This is not an isolated incident either. Two months ago, exactly the same thing happened in a different DD in Somerville MA, but there the manager listened and charge me the correct amount.

Please advice what steps I can take so that DD does not rip people off this way. I am sure a lot of people do not realize this or cannot do as fast calculations as I can. One lady in the queue even thought that probably food tax in MA was 8% - I wondered how long she had lived in MA.

Dev
Somerville, Massachusetts
U.S.A.