An invoice lists what you sold to a customer and shows the quantity and cost of each product or service.
User invoices if:
You sell goods or services and need to keep a detailed record of each sale. Invoices show the quantity and price or rate of each item.
You use sales tax, discounts, or any other item that is calculated as a percentage of charges. Invoices can make automatic adjustments to prices for you.
You use group items or payment items.
Each invoice stores information that you can draw from later when you
analyze your business. For example, if you want to know the sales income for
each item you sold, you can create a report (Sales by Item Summary) that
provides those numbers. The line item detail on your invoices makes this