Your money is green and they want it.
The purest will back off if YOU specify exactly what's to be done for
that puts him into an order writing role instead of Master vs Serf ala
"WE sold YOU a car?" The "alter of Mercedes" is long gone, as it should be.
My local M-B dealer offers three levels of oil changes: oil and filter
change, lubrication service and yet another variant. The differences
between them are only things like checking tire pressure, lubing the
throttle cable (car doesn't need it) etc. I buy only what's needed and
always ask for an estimate beforehand. And we get along fine.
And the sloppy approach just runs down the car - some owners don't care
but why then did they buy a Mercedes?