Remember, when dealing with someone in customer service to be polite and considerate. They are there to help you, and make shopping or using their company’s service easier. They are not trying to cheat you, and know what they’re talking about more than you do.
In summary: Shut up, stay calm, open your ears and let me do my goddamn job! I’m on your side, I’m here to help as much as I can, and that’s a lot fucking easier without you screaming at me that you know how my job works better than I do.