I think it would be easy to link either your credit card or your image (if they were doing that) to your MAC if you only made two trips to the same franchise with wi-fi on.

For example, imagine they have cameras that can image your license plate. You go there twice -- they have one MAC and two sets of possible plates. The odds that you and another person were both shopping at those times is pretty low. Now they have license plate, make/model of the car, can probably triangulate the wi-fi to know what you bought each time with reasonable fidelity...