It's about the coupon: You cannot use it on Pre-Order Titles, Add-On Content, or Ingame Items/Currency.
Currently on gg.deals it is coming up as a viable option for lowest price in some cases: See example for instance, Forspoken, says 25% off with coupon. But when going to the website, and adding to cart to continue and complete purchase it lists clearly that the game does not qualify for the discount coupon.
So I figured it must have been a bug on this site, if you could fix that it would be great! Seeing as no one wants to speed through checkout only to find out they paid full price 😡