Dear Mr/Mrs/Ms (name),
I am contacting you in regards to your letter dated (date), giving reasons for cancelling purchase (order number).
I apologize sincerely to you for the issue(s) which led to you cancelling your order and have brought the issues to management’s attention. Steps have been taken to ensure that the issue(s) does not reoccur.
As a valued customer your satisfaction is our priority. I hope you accept our humble apology.