Common size boxes rarely fit items perfectly. When shipping larger items, dimensional weight charges go into effect and an extra inch or two can cost you dearly. Even with smaller items, oversized boxes weigh more and require extra padding material.
The solution is to cut down the height of your box to be only as large as necessary. This can be accomplished by someone skilled with a box cutter by scoring the outside of the box at the appropriate height.
However if you're like me and can't cut a straight line to save your life, they make a very invaluable tool called a carton sizer. At $15-20, a carton sizer can easily pay for itself in one or two uses.
This is the carton sizer I use:
And here's a step-by-step video on how to use a box resizer