The purpose of this letter is to follow up with you regarding payment of invoice number(s) (fill in the number or numbers). As of (today's date) your outstanding past due balance is (fill in dollar amount), as detailed on the enclosed statement of account.
In order to keep your account in good standing and to avoid additional late charges, please remit payment in full immediately or contact me at (insert telephone number with area code) or (insert email address) by (insert a reasonable deadline) to make payment arrangements. If you have already made payment and feel this notice has been sent in error, please let me know.
Thank you in advance for your prompt attention to this matter. I look forward to your prompt reply.