I would actively listen to the customer's concern without interruption if at all possible. Next, I would summarize what was wrong with his order, etc. In a calm tone of voice, I would confidently assure him I'll do everything within my power to resolve the issue. If it ends up I need my manager's assistance, I would ask the customer if he/she had a moment so I may contact my supervisor. If customer agrees, I would introduce my manager, summarize the challenge and ask my manager how we may assist the customer from here. I would also attempt to privately signal to my manager permission for an additional % off or some other compensation suited to the complexity of the customer challenge.