Thank you very much ( very, very much) (ever so much) (most sincerely) (indeed) (from the bottom of my heart).
很（非常）（非常非常）（最真诚地）（确实）（衷心）感谢您。 Many thanks for your kind and warm letter.
感谢您友好而热情的来信。 Thanks a million (ever so much).
万分（非常）感谢。 Please accept ( I wish to express )my sincere (grateful) (profound) appreciation..