The user will likely take ~20 seconds to get their phone out, unlock it, log in to the bank app, confirm the transaction and set their phone down. The PSD2 API shows the transaction immediately (again, instant transfer being enabled is a prerequisite) and the seller can confirm that payment is complete.
I don't have a complete solution but this is all public information.
Barcode to read with your bank app (guide is in Finnish) https://www.finanssiala.fi/wp-content/uploads/2021/03/Pankki...
Example Bank API: https://op-developer.fi/p/psd2-info
The user will likely take ~20 seconds to get their phone out, unlock it, log in to the bank app, confirm the transaction and set their phone down. The PSD2 API shows the transaction immediately (again, instant transfer being enabled is a prerequisite) and the seller can confirm that payment is complete.