You can shop on EC sites other than Amazon.co.jp using your Amazon.co.jp account information.Depending on the store, you can also pay using your Amazon gift card balance.
If you have an Amazon.co.jp account, you can use Amazon Pay right away without additional registration or application installation.
Also, Amazon.co.jp does not share payment information with third parties without your consent.
Anyone with an Amazon.co.jp account can use it right away.