Anyone do a wheel exchange

[AmerLemans21]

My wife has ordered a new '06 X5 3.0. We both hate the 17" wheels that come on the truck and did not want the sport package (SP) suspension (and she doesn't care for the style of the steering wheel on the SP either) but likes the wheels you get with it. Our sales guy informed us that we can't just add the wheels that we would have to do a "swith out" with their parts dept in order to get different wheels. The parts dept would give us credit for the 17's and we would pay the difference for the 18's. #1) is this true that you can't just add different wheels when ordering the vehicle to begin with and #2) has anyone done this. I am curious as to how much $$$ credit you get for the stock 17's.

Thanks for any help here.