>Given that this runs atop Payment Required, doesn’t this mean that each API request would involve an extra one or two data transfers?

Yes, assuming its the first time you've interacted with an API endpoint and don't have cached payment requirements.

>Is there a reason why you wouldn’t pay ahead of time? I just understand why you couldn’t buy a few dozen/hundred/thousand dollars worth of credits, and wait until it runs low.

This would require manually integrating each vendor, which is totally valid if your agent performs a lot of a single type of task, but we suspect over time there will be huge value in agents being able to dynamically select tools / apis they want to use to accomplish their tasks, and dynamically pay for what they use