How might I best close a formal letter to a foundation who sponsored my travels?

In English I think:

1. Most Gratefully (respectfully)

2. In your debt

3. A very sincere thank you

It is a very important letter, as these people paid for all of my travels and I really want to tell them them thank you!

gratefully,
christine

Please,

You are helping me too much! But, I thank you. this is a very important letter to a very wealthy Turkish/USA family who seeks to promote Turkey in the USA.

By God, we need all of the help we can get. Our perception mortifies me!

Very best regards,

Christine