I hate sending pumps. Best way I’ve found is get a box big enough with room at the ends for a 1/4” scrap piece of plywood. If the box is dropped right the shaft will poke thru the box no matter what padding you use. Cut the plywood the same size as the box ends, you need two to do both ends. Get a can of spray foam insulation. Spray a small layer on the bottom of the box and quickly lay a big trash bag over the foam and set the pump on the bag. The foam will expand and form to the pump and also hold the plywood in place. After it’s dry I cover pump with another bag and add just a little more foam in a couple spots to fill the voids. Don’t add to much. Make sure no foam gets on the pump or shaft.
It’s over kill but it works.