YOUR INVOICE IS INCORRECT
I am in receipt of your invoice #[INVOICE NUMBER], dated [DATE, ex. June 8, 1998].
I believe there has been a slight mistake. [STATE NATURE OF MISTAKE, CITING AND ENCLOSING SUPPORTING DOCUMENTS, ex. As per our order #23345 and shipment receipt (which I have enclosed for you), we ordered and received only five A465 compressors, as opposed to the six invoiced.] Please check your records and inform me if I am incorrect.
I request that you send me an updated invoice at your earliest convenience so that we may process your payment.