selfish customers

usability issue with work-there is no way to say we are closed until we actually lock the door, and we cant lock the door if someone is in the store, so today, a women could not make up her mind on ordering a cake and she came in around 5:55 and we closes at 6. so she kept asking questions to one employee while the rest of us did closing duties thinking she would leave a couple minutes after 6. well at about 6:15 she is still in there and not interested in leaving and people are starting to come into the store thinking we are still open even though there is nothing left on the shelves-and then complain that there is nothing left. we need an "open close" sign!