People will always complain. The question is “Is the complaint reasonable?” If it is, then you had better be ready with your apologies, not your excuses. If it is not, then you are going to have to be diplomatic.
If the customer is king in your business then you cannot afford to mismanage customer complaints.
If you do it will affect your current business, repeat business, your reputation and your company’s brand.
Some people complain about nothing, all the time. Others keep their mouths shut and suffer in silence. Trouble makers simply don’t care at all. Some people make verbal complaints; face-to-face, by telephone or through third parties. Other people make complaints in writing; email, letter or even through a lawyer. If you have a particularly tough customer they might even complain about you on social media. What are you going to do then?