"I am writing to thank you for your excellent service and standards of work which I cannot recommend highly enough.
Your assistance in the design and branding of the documents and presentation package for a recent event where I was speaking was invaluable. Short timescales & vagueness from me were obstacles you comfortably overcame.
I appreciate the difference in appearance between amateur and professional work and without your support and expertise I would have looked the amateur. Instead you made me the professional."