I’m not trying to be argumentative here, but a plane is a surface that is commonly used by everyone. The STEP format allows for NURBs to be defined, but it also allows for planes to be specified as well. If you want to have a flat part made, you don’t need to use NURBs at all. As you’ve noted, a vendor that only accepts STEP files is prepared to deal with b-surfs, but they’re also prepared to handle flat planar facets as well.
Here’s an example of a SketchUp model sent to Xometry: