Dear Mr/Mrs/Ms (surname),
We are contacting you in response to your (correspondence/phone call) on (contact date) regarding your account. I am sincerely apologising for the treatment you received from our representative. This type of service is frowned upon by (company) and will be dealt with accordingly.
We value our customers and hope to continue to serve you in the future. We are asking that you allow us to show you the true (company) customer service and to make up for your disgruntlement.