Ah yes, the classic client scope creep. First panel: "Make me a portfolio website?" Simple enough, just slap some HTML and CSS together, maybe a touch of JavaScript. Second panel: "Now make me a simple store. How hard can it be?" Suddenly you need React, MySQL, authentication, payment processing, and whatever that circuit diagram is supposed to be. Probably the client's "simple" idea for a recommendation algorithm that "just works like Amazon's but better."
It's like asking someone to build a doghouse and then casually requesting they add an infinity pool and home theater while they're at it. Because you know, how hard can it be?