March 11th,2008(在此行最后,格式打不过去）
Dear ManagerX ,
I'm writing to tell you how very kind of you to repair our instrument for free which sent to your company several days ago.
We realized how much time you have saved for us.We feel deeply indebted to you and we really don't know how to thank you enough for your help.
Sincerely yours,（在此行最后）
XXX（同上）