When I was still in the banking sector, I can vividly remember one of the training's I had on managing complaints & customer feedback. I also observed a lot about how complaints were handled in M&S when I had a part time job while doing my masters degree. Now one of the differences between a good business & a 'not so good' business is the way they handle feedback/complaint especially when it is not a positive feedback. A good business uses the complaint to their advantage by the way they respond to it while a 'not so good' business ends up spiraling up the negative view of the customer and exacerbating the already bad situation.
Its as little as your opening remark... start with a 'Thank you' for your feedback... we are sorry you feel this way..... however.... then at this point, you begin with clarification or better still you let the customer/client know you will be looking into the situation.
I am fortunate to seat at the board of a private school and on one day, I received a complaint from a parent & when I called my staff to notify them, they go " These parents' always complain"..... my response was "If we treat every complaint this way, we will never make amends because we are not taking responsibility but we are pushing the blame back to the parents. Our response should be we will look into it, and if we are justified that we are not in the wrong, even then we should still be careful in the response style we choose".
Many businesses struggle with handling complaint/feedback but if a business succeeds in this area, it takes the business to a very strong position with competitors.
So if you are an entrepreneur or thinking of becoming one, make sure you have this sorted!