I had a customer who left me positive feedback but sent me an email that made me realize he wasn't 100% happy. He was really nice and was offering more of a suggestion. I asked him if there was something I could do so he was 100% happy, be it a refund, discount, whatever. He just wanted me to change something small, so I did, I paid for shipping both ways and we both ended up happy. What's funny was it was something I had been thinking about doing anyway and he helped me decide to start doing it.
I absolutely believe in my customer being completely satisfied with their purchase and in providing excellent customer service.