That’s very kind of you.
どうもご親切さま is an old-fashioned expression. I’ve never heard it from other native speakers. It even sounds like a satire – If someone talked to me like that, I would rather interpret it as “you spent your precious time and efforts on doing this, but I don’t need it at all, stupid moron”.
If you really appreciate someone’s kindness, just simply translate the English sentence as:
なんて親切な方なんでしょう！（しんせつなかた）= (Lit.) You are such a very kind person, aren’t you?