This shipping method is just my experience and has worked well for me. Out of the 60 (or so) finished models I've mailed out in the past few years, only one was damaged in transit.
To start, save shipping boxes, cardboard flats and used engine casings.
Throw away Chinese made shipping boxes. The made in USA boxes, such as Uline, are much stronger.
You'll want the rocket suspended in the box with air space and padding on all sides.
The bottom of the rocket is held securely to a raised platform on the bottom of the box.
Cut the platform bottom from cardboard sheet. The flat top should fit into your shipping box. The downward folded sides will raise the model up from the bottom of the box.
In the picture at the right I'm sizing the bottom platform for a slip fit in the box.
Notice the "X" in the platform center.
Run a line from the corners of the platform to make the centered X.
Trace around an engine casing.
Cut just inside the pencil line and through just the top layer.
Peel out the cut circle leaving the corrugation and lower layer.
Press the engine casing into the cut circle and remove.
Run a line of glue in the circle and press the casing in. Make sure the casing is vertical.
Follow up with glue fillets.
The casing will go into the engine mount of the rocket and support the model from the bottom of the outside box.