Two sets of cars, not seats. The seats would be pre installed. Dealers do not be assemble to taste (except for maybe small items like radio).

Chances are that the savings in doing it the current way are not passed on to the consumer but mathematically, technically they could be. Same like self-serve checkouts.

With software it is common to pay extra to unlock premium features. You don't pay and then download those features. This is the same concept.