Here's the clip with Rep. Pressley, which I'll note it's a bit different than how it was characterized in the article here on Ars:
https://youtu.be/D6-WaakMxjk
Summary of one portion of Ms. Pressley's questioning regarding Calibra Wallet:
Ayanna P: "Is it free to use the Calibra Wallet?"
Mark Z: "It's not a service that is available today, but assuming we can launch it, the plan is that it will be free."
AP: "No fee?"
MZ: "Congresswoman, that's the goal."
AP: "So there is no fee?"
MZ: "Congresswoman..."
AP: "Moving on. Since you are going to charge a fee, how does this help anyone?"
So they're bitching at him for a service that is not even available yet, but is presently planned to be free, and saying the problems for low-income folks in the whole country being "under-banked" is because it's too expensive and that's apparently his fault. Even the free services they're proposing are apparently too expensive for her liking. Zuckerberg's response here is not flustered. It's more like he's baffled that she could be so incredibly stupid and he's struggling to speak down at a level she can comprehend. He said free and no fees multiple times and she wouldn't listen, kept speaking over him, and then still got it wrong at the end and deliberately misrepresenting what he said.
