That seems crazy. Do you have blind spot monitoring and all that jazz?
Did you get a price just for the part?
There are only three options
1. The actual glass mirror.
2. The back cover - which is a "one size fits all" and needs painting to the colour spec of the particular car.
3. The whole mirror assembly.
As there were other bits missing/broken then it is option 3.
It may be that with it still being a relatively new model that all the other bits that are in the mirror housing (and there are a lot of them) will only become available in the future - or it is probably more a sign of the times with our "disposable society"!!