参考一：
Dear XX,
So sorry for this abrupt email. I am quite sorry to inform you that we made a mistake in our offer. Kindly check the enclosed correct quotation.
Meanwhile, attachment 1 is our official apology letter. Attachment 2 is our formal analysis report and our corrective action.
In order to make up for this issue, we would like to offer 3% discount.
Last but not the least, we are sorry for any inconveniences caused by this.
Best Regards
参考二：
Dear XX,
I am so sorry to inform you that we offered the wrong offer in the previous email. Kindly check the attached final correct version.
Due to my personal carelessness, I made a wrong calculation and I feel quite ashamed about this but I will never make the similar mistake in future.
In order to make up for this issue, I have done my best to make this order lead time 3 days ahead of time.
Last but not the least, we are sorry for any inconveniences caused by this.
Best Regards,