Dear Ms. Jane,
Thank you for your letter of 10 February returning the draft of the catalogue we propose to send to our customers.
I am very grateful for the trouble you have taken to examine the draft and comment on it in such detail. Your suggestions will be very helpful.
I realize the value of time to a busy person like you and this makes me all the more appreciative of the time you have so generously given.
Yours sincerely,