Power Transformer

Mebbe double box it with packing and a caution label, 9lbs isn't all that heavy.

No, just pad it so that, even if the box is dropped, the transformer doen't rattle around.

The weight seems insignificant, but the problem is it is concentrated in a relatively small volume. A large box and lots of packing helps, and if it has mounting feet that could be damaged or poke through the side of the box, bolting it to a piece of scrap wood might be indicated.

Crush test or burst test? One place I worked had these testers for cardboard that clamped the board between two large cast iron washers and a hydraulic pump inflated a small rubber diaphragm in the center (with a peak recording gauge calibrated in pounds per square inch) and it was pumped until the cardboard broke. There was no crush test that I'm aware of.

Swaddle it in bubble wrap and ship it.