Will I have to pay to use the Unreal Unlimited for commercial products?

Under this specific plan, there is no charge for Megascans or your Unreal Unlimited subscription, and you get unlimited access absolutely free.

Please note this unlimited Megascans access is licensed for use with Unreal Engine and Twinmotion only and treats Megascans assets as UE-Only Content under the Content License Agreement that you have accepted under the Epic Games account.

As per the EULA, as long as the end product is in Unreal Engine, you may use the assets acquired on the Unreal Unlimited plan for your commercial projects.