Fixed prices are popular because they remove the worry of a bill that keeps growing. They only work when the scope is clear, so every project starts with a short call to talk through your goals, your content and the pages you need.
From that conversation I write down what is included, how many rounds of revisions there are, and what is not covered. You see this before you commit, and nothing starts until it is agreed.
If your needs change along the way, that is fine. Anything outside the agreed scope is quoted separately before I begin it, so you always know what you are paying for.