When a simple website is no longer enough, it's worth thinking in terms of a web application. A web application can be a customer portal, an internal admin interface, a registration or booking system, a calculator, a data management tool, or any online tool that requires interactive functionality.
The advantage of a custom web application is that it can be tailored precisely to the business's processes. There's no need to adapt to the limits of an off-the-shelf system, and no need to juggle multiple separate pieces of software. The features, permissions and business logic can all be customized.