Re: it's up to you
Theoretically yes. In practice, well it gets a bit more difficult.
Yes, it's up to you to ensure that the number on the register is the one you're expecting. That said, a confirmation on the screen of your phone is entirely reasonable and justified.
Especially since this whole thing is going to run on code that you will never, ever see, much less be authorized to debug.
But hey, I'm never going to bonk with a phone, so I'll leave you to find out what happens when things go wrong.