I commissioned a piece of artwork by an acquaintance whom I consider a casual friend. I saw the piece today and was very pleased and I know she spent a lot of time and thought on it. She gave me the final price and I felt like it under-valued her work. Is a "gratuity" inappropriate? I have never commissioned artwork before so i am not sure of the etiquette involved. I want to show her that I value the piece and her thought, but I don't want to be patronizing or condescending.