Hi Distul, I think it’s safe to say that, on the current state of UE4; don’t give out promotional code for non-consumable items; non-restorable, as I believe (correct me if I’m wrong pls), UE4 yet have the functionality (blueprint/non-UE4 github) to fully support that.
However, I do want to point that, even Apple doesn’t allow you to give promotional code for an IAP, either it is non-consumables or consumables. They’ll only allow you to generate promo code if the app is paid-app(with/without IAP) and not a free app with/without IAP. Well, that’s at least what I experienced first hand in last Dec 2015, correct me if I’m wrong in Jan/Feb onwards 2016.