A letter of apology to customers for the mistake made in a recent invoice. It explains the cause(s) of such error and informs the customer about the rectification made.
You are sending a letter of apology to your customers to apologise for a clerical error in a recent invoice. You state that you have corrected the amount in the company's internal system and will proceed to issue the customer with a revised and correct invoice. You note that you have ensured that such clerical or accounting errors do not occur again, state that there is no excuse for such carelessness and thank the customer for their patience and for notifying you of the mistake.