the best advice that i can offer you is be nice. i know it sounds stupid, but hear me out. you need to show your concern so that the don't walk all over you, but be as nice as you can it will go a looooong way. when i have an irate customer i am less likely to bend over backwards for them. if a customer is upset, but understanding and calm i am going to do everything i can to rectify the situation. however, if they are unwilling to do anything about their mistake, then get pissed and let them know.