When I feel that way after a shop, I always try to say positive things about the employee. For example,
though there was a long line, and other customers interrupting our transaction, the CSR remained cheerful,
helpful, and courteous. Of course, I only say that if it's true. I may be wrong, but I would think a company that's having its employees evaluated would like to know if they remain professional "under pressure". Besides, it's so rare to find one who DOES do that, that I feel they deserve recognition.
I had a manager at 4+1 Guys not offer me any fries or a drink. I thought that was memorable.