Dear Mr. James,
Thank you for your letter dated 10 Feb 2xxx with a check of USD350 enclosed as payment for invoice No. 1234 of amount 380.
As you said in your letter, you have deducted USD30 as 8% rebate from the invoice. I cannot allow this as the invoice is more than 30 days old. Rebate is only allowed for payment within 30 days. Please remit the balance of USD30 and I will be glad to issue a receipt for your payment.