If it is plate then my experience is that it will be reasonably flat, having been cut from a chunk it would be understandable that it would need to be squared.
If it was cut from a extruded piece then that width approx 8" or 12" depending how it was cut would not be flat at all from my experience.
You could have that piece squared up and them surfaced on a mill, this should get it pretty square and flat no bigger then it is.
Unfortunately that will likely set you back another $30-$40.
They must have charged you a minimum charge at that price.
Are you in the USA?