Dear [Recipient's Name], I hope this letter finds you well. I am writing to sincerely apologize for the oversight on our part in not crediting your payment in a timely manner. We deeply regret any inconvenience or frustration this may have caused you. First and foremost, I would like to assure you that this was an unintentional mistake, and we understand the importance of promptly acknowledging and crediting any payments received. Our internal system must have overlooked this particular transaction, and we take full responsibility for the error. We genuinely appreciate your patience and understanding as we rectify this issue. We have immediately taken the necessary steps to investigate the matter and have already credited the outstanding payment to your account. Rest assured that we have implemented additional measures to prevent similar occurrences in the future. Please accept our sincere apologies for any inconveniences this may have caused you. We understand the value of your trust and confidence in our services, and we want to assure you that we are committed to providing you with the best customer experience possible. In recognition of our mistake and to express our regret, we would also like to offer you a [discount/coupon/gift] towards your next purchase or a [grace period/extension] for any pending payments. We value your business and want to make things right. Again, we sincerely apologize for any inconvenience caused, and we appreciate your understanding and patience. If you have any further questions or concerns, please do not hesitate to contact our customer service team at [phone number] or reply to this letter directly. Thank you again for your understanding and giving us the opportunity to rectify this error promptly. Yours sincerely, [Your Name] [Your Title/Position] [Your Organization] [Contact Information: Address, Phone, Email]