This is a very common question. There is some confusion around multi-domain services. But keep in mind that hosting and domain registration are two different services. When you purchase hosting… you purchase hosting… not domain registrations.
In other words, a hosting plan that can accommodate 5, 10, 15, 30, 100, or unlimited domains does not mean the registration of those domains is included in the price. What it does mean is that the installation of those domains is included — adding any number of domains to your hosting service at no extra cost.