That's the right face. I tried the fillet and got it to work (didn't think to try one so small (feature request: find largest possible fillet)), but Shapeways is still rejecting those edges. They require .028" minimum wall thickness, but it's hard to translate a corner into a wall thickness. I'm happy to push the face back .025" (based on their render that looks like it may be enough), or as far as it needs to go.