TurboTax Rip-Off On Amex Discount

The reason I decided to try TurboTax this year was the 25% discount on the federal return cost. In actual practise, this translated to about $5 off, but by the time that sunk in I was committed.

Having just filed, I feel even more ripped off by my inability to actual get my damn $5. The offer didn't come with a promotion code; to use it, you're supposed to access TurboTax through a special page on OfficialPayments.com. (This special page: https://www.officialpayments.com/promotions/turbotax/amex/index.html) When I started my return, I initiated it at that site. When I went back to finish it today, I want straight to TurboTax. Come checkout time, no discount. So I went back to the original site and saw the note that you need to "Remember to access and complete your return through the TurboTax links on the American Express Web site." Being a dutiful little discount hunter, I clicked the link, re-stepped through TurboTax to the checkout ... and again saw no discount. GRRR.

At this point, I decided further chasing was not worth the $5 and surrendered. But if anyone does manage to use an OfficialPayments discount: What's the trick? Do you also need to open an account there and log in through that? (Something NOT MENTIONED BY AMEX AT ANY POINT ...)