An apology for error on invoice

OUR APOLOGIES FOR THE ERROR ON THE INVOICE

Mr [CONTACT name],
Our apologies for the error we made on our invoice dated as of [DATE]. You are absolutely right, we have just the fault. I entered the amount in our books of account as being [amount] (Fortunately the amount [amount] was not recorded in the book of accounts).

I could not realize this error, I'm happy that you you to be seen. Thank you for your letter.