Since you said "Mold" I think the only option is to re-create it at a lower polycount (If for some reason it need to remain a poly model). By the way, Solidworks utilizes Mesh data? What is the source?
The only way to reduce filesize on an STL is to reduce polycount, and this also morphs the shape. For a 2 part mold, I would think this to be a bad thing.
You can point your customer here:
And refer to the "resolution" area. You can have them dice up the model at a lessor resolution. But if Solidworks is dicing it up, then dont use STL. Use a NURBS format and dont ruin the model... If they are just importing STL reference data, and exporting that to you, then you may go back to the source to get a lower polycount.