Firstly, I apologize fot this incident. I understand your frustration, recently we have gone through some changes in our invoicing department in order to improve the process in the past year but have discovered this error recently from the internal audit. Our invoicing department did not multiply the daily rental rate by a number of days rented for materials. I am aware that this is a basic but very serious error and I am very embarrassed by this. The problem had been fixed since , but we understand your frustration about the error and the delay, and are willing to accept any penalty for this invoice. Please let us know how you would like settle this outstandings