You have an idea and want to implement it. You think it should have been in the project by design, but forgot to include it in the project description prior to being given a price. If it was not present when the pricing was calculated, then the price will be adjusted with anything and everything you decide to include / change at a later stage.
It’s the same the other way around. We won’t tell you that features you paid for won’t be implemented, because we just though so – and we would expect to get paid for features not requested when the initial price for a project was given, based on communicated and requested features.