I tend to use appreciate in my letter endings. Such as: "I appreciate your help".
I wonder if it is too much formality?
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
If it is to someone you don't know personally, it is not too formal. For a friend, usually a thanks will do.
It's not more formal at all. It does read like a deeper level of thanks than a simple "thank you" or "thanks" but I doubt anyone will think anything of it.
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.