What features do you want your website to have? What do you need for functionality?
It is important to think this through before the process of building the website begins. Additional features that are not agreed upon in advance can drastically make your website more expensive than planned.
After all, it is not always that you are aware of all the needs before the website is in the construction process, they can sign up along the way, and that is perfectly okay. In this case, the price of the additional services will be agreed upon as they sign up.
However, the more that is in place before the process starts, the better. The features you need also determine the choice of system/platform your website will be created in, so if they are not at the beginning of the process, it may not be integrable with the system/platform chosen.