Shirkrin -
Thanks for your questions. No royalties are owed on products that are truly free (i.e., no ad revenue, no in-app purchases, etc.). If a game is used in a commercial, no royalties are owed because the license excludes linear media from owing royalties. When you say an App Store promo, I’m not sure what that would entail.
Even if your product is free, we still ask that you notify Epic of its release here.