The designs may be identical or different, but they must be have the same layer thickness to get this discount.
Some people find that a dip in mineral oil helps to reduce porosity.
Sprues can be wax-welded onto the piece by hand, or it can be very convenient to build a sprue stub onto the CAD model. Like wax, the material can be polished with alcohol or lighter fluid. Unlike wax, it is also somewhat water-soluble. It will not be affected by normal investment procedures, but it should not be wetted or soaked or left out in the rain. Free material samples are available if you would like to experiment.