I established a fixed price with a client for a website design project. I got underpaid because I couldn't show any experience in the area of the project since I didn't had permission from my other clients to show my jobs for them on my portfolio. I accepted this just because the client told me there will be more work up ahead. Now, we're near the end (the site is up and ready) and I've already been paid, and still the client wants to add more features to the home page of the site. How can I tell my client that we are already out of scope for the agreed payment amount and that implementing this new things means that I'll have to charge him for it?