An interior design invoice is a form containing descriptions of completed services by an interior designer and the associated costs. The invoice will show the client how much work was done on the interior of their home, office, or other construction and how many materials were used for the job. The interior designer will keep a detailed record of all expenses during the project and add them up to create a grand total on the invoice. Payment must be made by the customer as per the payment due date written on the invoice. The interior design company (or individual designer) must include their contact info to ensure the client knows where to send the payment.