I know e-mail has a faster round-trip, but they also don't ask you to confirm snail mail.

I think it would be quite annoying to have to verify my purchase everywhere, just like how I don't wanna sign up to every single merchant online. Let me purchase as guest without having to enter OTPs.

Email isn't guaranteed to have a faster round-trip. https://groups.io/email-provider-status -- sometimes goes into hours of latency.