Me: "No ma'am, we can't track an Amazon RMA number, you'd have to contact Amazon for that. Since this receipt is dated yesterday, the items have already left, as everything leaves the store daily."
Customer: "I don't like your tone, I don't appreciate your attitude, I don't like you."
Me, smiling, pleasant customer service voice: "Well, you don't have to speak with me, you're free to leave the store anytime."
And now I'm in trouble and will be getting "talked to" by the boss when he comes in later today. Quote my manager "you can't be mean to customers." But apparently they can be mean to me? Sorry, no, I draw the line at tolerating someone sneering directly at my face about the things they dislike about my existence.
The worst thing about working customer service jobs is that one suitably awful customer can, beyond just making your day worse, actively put your job at risk. Especially in America there's an expectation that customer service workers be punching bags for whatever irritations are on people's minds that day, and that they lose their jobs if they express any sort of human emotional response to being harassed.
Personally I believe all customer service workers should have a button under their counter, and when they press it, it ejects the customer through the roof on a Dr. Seuss style springed platform.