We don't really like the selection either, but they don't install anything if you didn't buy it from them, do they?
Normally not. If purchased through the SHK/General Contractor, there is a decent margin via kickback from the sanitary showroom, where you almost always pay the retail price.
We did it this way: Chose a washbasin from Villeroy & Boch, bought it online and stored it. Gave the installation drawing to the SHK and had the connections and wall reinforcement adjusted to fit the mounting. Got back about 30 euros for the standard basin :p
And after moving in, hung the basin plus the cabinet ourselves.
Basin with cabinet would have cost around 2500 from the HSK, so it was still 900 this way.