We've attached the overdue invoice to this letter. Please remit the payment within 2 business days.Prompt payments enable me to meet my commitments and continue offering you the best of my services. Follow these 10 tips to write overdue payment letters for requesting outstanding payments from your late paying customers. What should be included in a demand letter? Discover what an overdue invoice letter is and learn steps and tips to help you write and send overdue invoice letters to your clients. What should be included in a demand letter? Please let us know the status of your payment as soon as possible.