Used moving boxes are great for shipping purposes even though they tend to be larger than what we need most of the time. There is a way to cut them down if need be. If you have excess empty space at the top of your box, it's a prime candidate for cut down. Follow these steps for a perfect job.
Pack and fill the moving box as normal, then use a ruler to measure the distance between the top edge of the box and the layer of fill.
Using a sharp knife, cut each corner from the top down, going as deep as your measurement. When you're finished, all four cuts should be the same length.
Next, use a ruler to draw a straight from the bottom of one corner cut, across the narrow side to the other. Do the same thing on the opposite side and then the two long sides.
Use your knife to score the outside of the cardboard along the four lines you just drew. Be careful not to cut all the way through.
When all four lines are scored, place the ruler flat inside the box at the level of one score. Fold in and over the ruler for a straight edge. Repeat one each side.
With the sides folded in you have created four flaps. The flaps on the narrow sides should not overlap; on the long sides they probably will.
Fold one of the long flaps in, then the other on top of it. Use your knife to cut off the access from the flap underneath. The two flaps should now meet edge-to-edge.
If you've done everything correctly, you should now have smaller box for shipping. A smaller box means less money out of your pocket.