Dear Mr. Hunnicutt,
I am writing to apologize for the incorrect invoice you received from us. Owing to a breakdown in communications, we did not credit you with a 10% discount, as agreed.
We shall send you the correct invoice as soon as possible and make sure that such a mistake will not occur again. Please accept my apologies for any inconvenience we have caused.
I am truly sorry that I cannot attend the project meeting to be held on next Tuesday afternoon.
The reason is that I am going to take a business to Shanghai by plane that day, so I will be unavailable for the meeting. However, considering the importance of the meeting, I will be grateful if you could send me a brief record of the project progress by E-mail. I will check the record and send my feedback to you.
Once again, I am sorry for any inconvenience caused. Hope you can accept my apologies and understand my situation.