Firm letter on past due account

Dear On (date) we called your attention to your account balance in the amount of $ According to our records we have not received either a reply to our letter or a remittance to clear this account. We extended credit to you on your account in good faith and expected payment under our normal thirty day terms which we feel are most reasonable. So that no further action on our part will be necessary, we shall expect a remittance by return mail or the courtesy of a reply to our letter as to why payment is being withheld.