A "proper invoice" refers to Question 28 options: An invoice that is considered to be correct by both the purchaser and the product/service provider. It is a commonly used term in industry. An invoice that is considered to be correct by both the purchaser and the product/service provider. It has a special meaning in the Construction Act. Is an academic term only. When a product/service provider provides an invoice, it is always considered proper. An invoice that has all taxes applied correctly