Thank you for the letter informing me of your decision to choose (Name of Service/ Product) to fulfill our current need.
I am very pleased that my research and advice on the range of (services/products) available provided helpful insight and stimulated a constructive discussion that led to a mutual agreement.
I am confident that your choice will lead to the desired results.
If I can be of any assistance in inquiries or discussions about how to meet additional needs or goals, please don't hesitate to ask. It would be my pleasure to further contribute to the continued growth and success of the company in any way I can.