How can I proceed with e-filing my return although I've already opted to mailing it in? The file by mail form states I can do this but I can't figure out how.

As long as you have not mailed your return yet, you can change from filing by mail to electronic filing.  

Take a look at the following TurboTax article for step by step instructions:  How do I change from mail to e-file in TurboTax Online?

@napostol12387

Thanks so much, this worked!