I feel obliged to offer this warning:
The automotive industry is extremely regulated, and if anything goes wrong with their vehicle which endangers the life of the vehicle occupants or other road users they are liable.
This is just my opinion, but it comes from someone who has designed and built mission/safety critical embedded microelectronic systems: Do not interfere with the control systems of your car, and definitely don't trust your life and the lives of others to a third party component which will not have been through the rigorous testing and legal requirements of the car's original systems.
Likely this "box" will probably be built around a cheap ESP32 (probably a consumer standard unit, not an industrial or automotive standard), which could fail at any time due to extreme environmental conditions, the software will have been coded to an unknown standard by an anonymous coder, with unknown edge and corner cases, and the full range of interactions with the rest of the vehicle systems will not have been fully tested.
If you want more range, the best way is to learn how driving style, loading, and route planning affect the power usage of your vehicle.
Apologies for the ranty nature of this post, but cars are dangerous.
-Edit- Also reading the web site, it appears all this thing does is reduce the apparent position of the throttle peddle... I don't need to spend the better part of £300 just to be a bit lighter on the "gas"... Also have you tired the cruise control limiter?