Some freight yes, some freight yes, probably not. Fruit and mail needs to go very fast and is often shipped by airplane. There are a few cases where regular shipping doesn't deliver on time and so an emergency order is air freighted in - at great expense.

I doubt that they can get their costs lower than airplanes. They need to buy land all the way from point A to point B, vs only at airports. Airplanes also run at a partial vacuum (30000 feet) by nature. As such it is hard to see anyone using them instead of something else. Where speed counts airplanes are faster, where speed doesn't count ships and slow trains in air are a cheaper.