2. The world is full of dickheads
Here's the thing about "nice people:" no one is nice 100% of the time. The littlest thing can cause "good" people to turn into complete and utter assholes. Someone cuts them off on the street, their cat has a gallstone, or maybe they just really have a stick up their ass. Sometimes, these people buy things, and they take their shitty attitude with them. Don't let these types of people ruin your day. You can't control that, and it's a reality of life. Your best bet is to get them out of your life as soon as possible, so provided they're not being overly hostile and making completely unreasonable demands, just put on your best shit-eating grin for a few minutes, get them out of the store, and then move on with your life.