Write a letter seeking apology after the cancellation of order.

Dear Sir,

This is to acknowledge the receipt of your letter of [date] which explained the reasons for canceling your purchase order. I am deeply sorry about the misunderstanding that has led to this cancellation and have taken the matter up with management in order to ensure that such a problem does not occur again.

We would like to reiterate that as one of our valued customers, your satisfaction is one of our primary concerns.

Kindly accept our sincere apology.

Thanking you.