I have a 20% discount for the cost of the filing which ends today. I am not ready to "start" my return as I need an updated T4. I see no option to pay when I log in.

You would need to start a return, and fill in the My Info section. Then when you get to the Auto-Fill section, the side menu will unlock and you can select File > CRA Online mail. At that point, you'll be prompted to pay.