An invoice letter to a client is a formal document that outlines an agreement between a business and its customer. It is sent to the customer after goods or services have been delivered, and should include details like the amount due, payment terms, and a description of the goods or services provided. By sending an invoice letter to a client, businesses can establish and maintain a professional relationship with their customers while protecting both parties in the deal. With a clear and concise invoice letter to a client, you show your customers that you value their business and deserve to be paid on time.