TurboTax is advertising a single return discounted from $19.99 to $16.99. I'm at payment section and it is show $19.99. How do I get the discounted price?

Please check the advertisement. These offers usually have a time limit, and you may have gone past that date. Or there may be a special link that you need to use to sign in to get the discount.

There was no best before date listed on your email advertising!  I opened my taxes less than 2 weeks following receipt of the advertising.