Methods, systems, and apparatus, including computer program products, for implementing a software architecture design for a software application implementing manual invoicing. The application is structured as multiple process components interacting with each other through service interfaces, and multiple service operations, each being implemented for a respective process component. The process components include a Customer Invoice Processing process component, a Due Item Processing process component, a Payment Processing process component, an Accounting process component, a Project Processing process component, and a Balance of Foreign Payment Management process component.