Why wasn't my 20% discount offer applied when I paid for my tax return?

I received an email from another user that gave me a link for a 20% discount off of TurboTax Federal return products.  This person also will get a $10 gift card.  I used the link in the email to start my return.  Why wasn't the 20% off applied to the invoice I just paid?  I have not yet electronically filed my return.