I e-mailed McDonald's on Wednesday and actually received two replies - a form letter from McD's itself and one from the store owner.
Here's the one from the store owner:
I received the complaint that you emailed to McDonald's Corporation concerning your visit to my restaurant on Wednesday, November 24 yesterday December 1. I apologize for the inconvenience you described with the treatment at my restaurant. I am not sure how the price you mentioned was arrived at, and I have checked to make sure the price programmed in our computers is correct. Our shakes have the syrup added as the ice cream is put into the cup automatically. It sounds as though there was a problem with that function at the time of your visit. I am sorry that you did not ask for a manager at the time of the incident in order that we could have taken care of the situation at that time rather than having it carry on for a week. Once again, I do sincerely apologize for your experience and will directly speak with the employee you described. Thanks for taking the time to contact us.
I'm impressed - Taco Bell never responds to my complaints about the tomatos...