My latest client project ballooned with extra requests, and I'm unsure how to handle the cost.

I was doing a logo design for a small business. After we agreed on the first draft, they asked for three more versions with different fonts. Then they wanted matching business card mockups. I didn't charge extra because it felt like part of the process. But now they're asking for social media banners too. I'm confused about when to say no or add fees. How do you guys draw the line with added work?