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I simply keep most packing material from items I buy and reuse it when I ship.

Most items from commercial sellers comes packed in very high grade pillows, foam, etc.
If you have to buy that stuff, it can cut into profits quickly.

I also keep stiff cardboard corner bracing that is sometimes used.
That stuff is great to keep a box from getting crushed.

If shipping a long item, make the box a triangular prism.
MUCH sturdier than a tube, or even a rectangle.