Simply replace the unfilled areas such as "" and "[date]" with your information.
This is a friendly reminder from [company name], that your account is currently [# of days] past due. Your current balance is [$]. If you have already forwarded payment, please ignore this letter.
If you have already sent payment, please disregard this notice. If you are unable to pay the amount due, please contact us immediately so that we can help you resolve this matter.