Slowing Down the Express Line
Answer: Your frustration is understandable but let us examine your options.
Reprimanding your fellow customer will most likely be an exercise in futility because he or she probably doesn't take seriously the sign limiting the service to six items. Calling this violation to the attention of the clerk at the express checkout counter will probably not achieve much either because she is in the middle of making up the bill for her current customer and has no time to listen to your complaint.
The only reasonable option is to let the supermarket manager know that his establishment will lose credibility if the limit announced at the express checkout counter is not adhered to, and that it is therefore important for him to enforce this rule by stressing to his clerks that they must turn away from that line anyone who exceeds the six-item limit.