Importers to render invoices except in cases of retail sales.

Invoices shall be rendered by importers and distributors to all purchasers from them of motor vehicle fuel within the District of Columbia except in case of retail sales. Said invoices shall contain a statement, printed thereon in a conspicuous place, that the liability to the District of Columbia for the tax herein imposed has been assumed by a licensed importer named in said statement and that the importer has paid the tax or will pay it on or before the 25th day of the calendar month next succeeding the purchase.