Dear …
On behalf of __(company name)__, please allow me to extend our deepest apologies for __(briefly describe what happened to the customer)__. It was clearly a mistake on our part for failing to __(briefly describe what the company’s failure was)__. Rest assured, it is our full intent __(to do what we can to make up for this oversight/to reimburse you for any additional and unnecessary expenses you may have incurred as a result of this oversight/etc.)__.
__(Name of Company)__ is based on professionalism and when we fall short of that mark it is of the greatest concern to us. We are aware that our customers are our greatest commodity and you are certainly one of our most valued customers, __(Name)__. I want you to know that we are committed to conducting a full investigation into this matter to determine how the error happened in the first place, and then to implement all the necessary controls and measures to ensure it never reoccurs.
Mistakes like this are regrettable to be sure and I want to thank you for both your understanding and your willingness to continue working with us. We do value your business, __(Name)__, and we look forward to continuing to build a strong business relationship with you.
Sincerely,