So I look at it like this:
Are the parts OEM? And if so how often does a BMW really install those parts vs the port. Also you will be the mercy of the dealer hourly rate and they wont warranty a bad install as its not their parts they sold (assumption).
There are so many reputable installers all over that can do this. I have no idea what RRT is but Im sure there are others. Installing a diffuser is not like doing warranty or service work. Installing side skirts are plug and play.
Basically I have never taken my cars for any mods like this to a dealership.
Just a simple search yielded some other options:
https://www.google.com/search?sxsrf=...4dUDCAk&uact=5
At worst drive your car over the Left Coast and have Ben do it
Sorry Ben I had to
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