Relativists say, "no, not everything is objective because each situation is different or a person might think a certain action is better morally better than another" when in reality, for each relative situation THERE IS AN OBJECTIVE TRUTH. So yes, the objective truths do change "relative to the situation," but there still is an objective truth to each of those situations. Relativists have it wrong because they think that there isn't just one answer to anything, when in reality, there is. In both situations where you treat the cashier with bare politeness, you are doing the wrong thing. You should treat the cashier with kindness because it is virtuous and it maximizes the total happiness in the situation. It doesn't matter how you are feeling, there is an objective way to treat the cashier: with kindness.