I have two on line sales businesses. You can offer the very best customer service, you can bend over backwards to make sure that you go beyond even what is reasonable to ensure customer satisfaction.
You can receive thousands of commendations from very satisfied customers. Despite all of that, there will always be one sad low life T$at who doesn't have sufficient drama in his life, and wants to make a problem for you. That is the way the internet works. All you can do is ignore it, any response will simply fuel it further.