As of November 2020, the Sandbox app does not support balance charge, so there is no way to charge existing users with PayPay balance.
By registering a new PayPay for Business account by following the steps below, 3 test users for Sandbox will be issued, so please use that account.
1.Create an account with PayPay for Developers
Access PayPay for Developers (https://developer.paypay.ne.jp/) and press "Sign up"
※In case of Japanese notation, please switch to "English" by switching the language.
Enter the email address and password you want to register and press "Agree and Create Account".
A one-time password will be sent to the registered email address.
Enter the received one-time password and press "Verify".
For the service logo screen, click "Continue" to proceed.
Make sure that all payment methods are checked, and click "Register".
Click "Start" to proceed to the dashboard.
2.Log in to PayPay for Developers
If the following screen is displayed, you can log in to PayPaty for Developers with the created account.
Click the "Test User" tab to display the list of test users.
You can check the password by pressing the "eye" icon.