I'm using "gmthanks20", if you see a typo there let me know, as I really do want to the discount. It did initially work, then I added another product to my cart and it stopped working. So I'm assuming a bug and not a typo.
The discount code is not eligible for subscriptions, preorders, backorders, non-Paizo PDFs and gift certificates, so if you added a product that is one of those, the code won't work.