This is a letter used to let your customer know that you have approved their request for a monthly statement. It's an example you can modify to fit other special requests made by a customer as well.
This letter can be used to advise a customer that the cash discounts they've taken are not valid, since they paid beyond the period you allow a discount to be deducted.
The point of this letter is to let your customer know that, while it has taken an unearned discount, you will allow it, but you expect adherance to your terms of sale in the future.