Simply replace the unfilled areas such as "" and "[date]" with your information.
This is to acknowledge receipt of your letter of [date] in which you set forth your reasons for cancelling your purchase order, # [number of purchase order].
I am very sorry about the misunderstanding that led to the cancellation and have taken the matter up with management in order to ensure that a problem of this nature does not occur again.
As one of our valued customers, your satisfaction is one of our primary concerns. Please accept our apology.