When we asked about the pad, our supplier stated that 1/8" per foot of slope was fine. It didn't need to be absolutely level, in fact, having a bit of slope helps when there are dips in the surface.
As dronning stated, however, if the contractor states there's no guarantee the pad will be level and flush, then it's the wrong contractor. If the pad can have forms on 2 sides, a screed board can be used to level and surface the new concrete. Else, an intermediate leveler board or some other method can be used - in short, a competent contractor or even an observant DIY'er can lay a flat pad. Concrete shrinks ever so slightly as it cures but it doesn't dip a 1/4 inch in the thickness.
If you've got the room to add a couple three inches to the top of the pad to level it - add new forms and lay a new pad on top. If not, it looks like the leveling compound is your only choice.
Good Luck!