iPhone shuts down when double-clicking Paybox app

The app developer may not have updated their software to ensure compatibility with the iOS version your device is currently running.

If you encounter any challenges with third-party applications, the best course of action is to directly contact the app developer for further assistance. Here's a brief guide on how to do it:

1. Identify the app developer.
2. Visit their website or app store listing.
3. Look for their contact information or support channels.
4. Clearly describe the issue you are facing.
5. Provide any necessary information they may need.
6. Follow their instructions and provide feedback.
7. Be patient and follow up if needed.

By reaching out to the app developer, you increase your chances of receiving accurate and personalized support for the challenges you're experiencing. To contact an App developer --> How to contact an app developer - Apple Support

Go to Settings > Accessibility > Side Button, and choose the Slower option. You may not be double clicking the button fast enough depending on your setting there, so instead of initiating the payment , it just turns off the screen.