Am I missing something (always a possibility)...
What's the issue with this?
Manufacturer builds car with all the options in it. If the buyer specs all the options, they're all turned on and active for lifetime of the car.
If they don't buy some of the options, but then decide they would like some, they can have them enabled. Either at some sort of monthly cost or a one-off lifetime cost in-line with what the build cost option would have been? Seems entirely reasonable so long as nobody takes the piss.
And clearly you'll just be able to go to eBay and do it for pennies like I've done with Apple CarPlay on the M2...