[TheGrumpyBrit]

Which honestly doesn't sound relevant to the purchaser. My understanding is that the Monroney sticker stipulates exactly what features are included in the vehicle, as provided by the manufacturer. By purchasing the vehicle, the customer has paid for exactly those features.

This is like them saying "Oh, we forgot to charge your dealer you for the premium interior and lighting" (which is listed on the same sheet as a $5000 option) - your response would quite rightly be "Tough, I bought it from the dealer at the agreed price based on the features listed, it's mine now"

[gambiting]

Absolutely, and as others have already pointed out, it's on the dealer to put this right. Either refund the customer, pay for the upgrade, or take the car back.