The place where you bitch, moan and cry to the unsuspecting checker who's just trying to provide you a service. You shouldn't do this as usually they
don't care about what you're about to say to them about your
guacamole jar being out-of-date. They just want to know if your day was alright and if your shopping
experience was fine. In all cases, they can just call someone to get you a new one. Be polite please, and learn what the check-out line is actually for: to pay for your products and get them bagged in an orderly fashion.