You must simulate the user interaction of selecting a posting option.

Handler methods are used in tests to simulate user interactions, such as responding to dialogs, confirmation messages, or option selections. Since you need to simulate the user interaction of selecting a posting option (Ship, Invoice, Ship & Invoice), a Handler method would be the appropriate choice for capturing this kind of user input.

Normal attribute and Test attribute are used for marking methods for test execution, but they do not simulate user interaction, making Handler method the correct answer.

For more details, see the Microsoft Docs on Test Codeunits and Handler Methods.