Online invoicing and time tracking service FreshBooks has a revolutionary new feature for developers using its FreshBooks API: an opportunity to make money through the company's current customer base. Through the program, developers can charge users a monthly fee, which is tacked on to what subscribers are already paying FreshBooks. The solution, another revenue stream for FreshBooks and a rare one for developers, marks a potential new approach for software-as-a-service (SaaS) companies and one FreshBooks thinks has a bright future.