However, you can go to your service center and buy the parts for $150 less and install them yourself?
Sure but at that point you only have the equivalent to the accessory hitch option, which was only $250 when the vehicle was ordered. I doubt they will enable the software towing option if you install it yourself. Or at a minimum there would be a labor charge, and I would hope that they inspect the install to make sure it is up to spec before enabling the software. (Additional cost.)
From past history it seems like they normally charge at least $500 to change a software option.