Screenshot Step-by-step
In the App Builder, go to 'My Account'

Click 'Unsubscribe'

Delete Sandbox Commerce App from BigCommerce
Click 'Apps' in the lefthand menu

Click 'My Apps' in upper-righthand corner of screen

Click 'Uninstall'

In the App Builder, go to 'My Account'
Click 'Unsubscribe'
Click 'Apps' in the lefthand menu
Click 'My Apps' in upper-righthand corner of screen
Click 'Uninstall'