I had a offer from Lending Club for 20% off for buying Turbo Tax. I forgot to add that to the checkout when I payed for it the other day. Can you honor the 20% discount?

Unfortunately, since you've already submitted your filing, there isn't any way to remove the 20% from your charge.

For you to have been able to apply that, you'd need to Clear and Start over any progress you've accomplished, and be sure to sign out of your account. Go to the page offering the discounted price, and make sure you go to TurboTax through their page.