Which item is NOT typically included on an invoice?

An invoice is a formal document issued by a seller to a buyer that outlines the details of a transaction. It typically includes critical information such as a unique identification number to help track and reference the invoice, the amount due for the goods or services provided, and the customer's contact information to ensure accurate delivery and communication.

While discounts can be mentioned on invoices, they usually pertain to the current transaction rather than offering discounts on future services. Therefore, including a discount for future services is not a standard practice on an invoice, making it the outlier among the options listed. Invoices focus on the current sale's details rather than potential future discounts. This reflects the purpose of the invoice, which is to inform the buyer of their current financial obligation rather than promotional offers.