Unpaid Invoice

To: {Company Name}
      {Address}
      {City, State, Zip}

Dear {Recipient},

This letter serves as official notice that we are still missing payment on an invoice dated {date}. The invoice number is {number}.

You sent a payment on {date} for invoice number {number}, which we appreciate. We believe it was just a mistake that you did not also include payment for invoice number {number}.

If you would like us to resend a copy of this invoice, or if you have any questions or concerns, please contact us.

Thank you,

{Sender}
{Contact number}