> How do you validate I've paid and let me through with a static QR code for something like the metro, or a ticketed event, or even a coffee? How do you know th

> How do you validate I've paid and let me through with a static QR code for something like the metro, or a ticketed event, or even a coffee? How do you know they paid the right amount? I turn and show you my receipt, and you hope it's not faked? By and large when I've used these systems, the vendor just checks they received the correct amount of payment in their app. It's largely immediate