Why should I be bitter?

Not having the greatest week. Thankfully, it's ending soon. Wednesday, I volunteered at the store (by the way, buy Fair Trade items!). Like I mentioned before, customers scare me. I rang up a woman and accidentally said the wrong amount (darn chiffres), but quickly corrected myself. That should have been a sign that I was about to have a bad afternoon. Soon after she left, an older gentleman walks in and starts to admire some statues of animals. I wasn't really paying attention to him until he turns around and says something to me. I didn't understand so I go and find another volunteer to help him. He tells her that he had said, "This paw is shorter than the others." Ah...okay...now I understand. Then he asks her if I am English or American. I answer that I'm American. She goes back to what she was doing and he brings over 2 statues to buy. I ask him if they're a gift. He responds that they aren't. I ring him up. He gives me a 50. I accidentally give him too much change, but don't realize it right away. As I'm on my way to get some bubble wrap, it hits me that I made a mistake and turn around to correct it. He hands me back the money without saying a word. I apologize and give him the correct amount. He starts to point out that I screwed up. I explain that he now has the correct amount and point at the receipt, several times, to help explain. He's not happy about that and continues to tell me that I make a mistake. Then he yells "Madame" to get the attention of one of the other volunteers. He then explains the situation to her. What, am I 5? Is she my mother? Are you trying to get my in trouble? She looks at me and I look at her and shrug my shoulders. As the other volunteer and I are working together to wrap up his stupid statues, he asks her how long I've been living here. I respond to him that I've been living here for a year and that I understand him...asshole. He says "good-bye" as he leaves. "Don't let the door hit you on the way out", I dreamt of responding.