Cashiers with a printer

1. Don't charge again: the sale is done and so are its vouchers.
2. Tap “Printer failed: show the QR” and have the person take a photo of the QR. They can pick up with that photo just the same.
3. Then check that it has paper and battery, and that it's on and close to the phone. Turn it off and on; on the phone, pair it again.
4. If the paper came out blurry or cut off, tap “Print a copy”: it comes out marked COPY and it's the same voucher. Tear up the bad paper.

A paper lost later isn't reprinted: the organizer voids the voucher and issues another.