I tend to use appreciate in my letter endings. Such as: "I appreciate your help".
I wonder if it is too much formality?
How formal the word is depends mostly on the context you use it in. I will sometimes casually say, "appreciated" to my friends if they do something for me, and that's very informal. Letters are usually slightly (but only slightly) more formal than direct speech though, even between friends. "I appreciate your help" would not be out of place in such a letter, though I would word it with a slightly less-formal, "I appreciate the help." "Thanks for helping me out with that" would also be a good option.