@rosemarie Just to make sure we're on the same page, checking the box alone will not effect any payment.  You will actually have to file another extension with the box "Electronic payment of tax due with extension" checked.  Good luck.

Thank you for the update.  The system won't allow me to e-file a second extension so I'll just need to mail in a check to be safe if this is the case.

Better still, consider asking your client do Direct Pay over the internet.  That's an easier and more assured option than sending a physical check through certified mail.