Yes Bundling In-App key with app is not proper solution and it will bring security risks and I'm not mentioned bundling it in my comment. My Ask is can we just make the offer signature as optional thing it really doesn't makes sense only adds additional step where we already showing the offer based on our app logic.

This is required for our reporting purposes mainly. Additionally this will help us to show users about how much they saved this current cycle.

Have you checked in Settings > Developer > Sandbox Apple Account (Found at bottom of the screen) > Initiate Transaction This will let you enter the promo/offer code and redeem it.