APOLOGY FOR LATE PAYMENT
In reference to your invoice #[INVOICE NUMBER], enclosed, please find our check #[CHECK NUMBER] in the amount of $[AMOUNT].
I do understand that this payment is late and I hope you will accept my apology. [BRIEFLY STATE REASON, ex. The recent brick-layer strike has adversely but only temporarily affected our liquidity.] We do, however, look forward to continued business with you.