From the link I posted:

Quote Originally Posted by config
Larger packages - added packaging percentage:weight
What is the weight of typical packaging for Large packages?
Example: 10% + 1lb 10:1
10% + 0lbs 10:0
0% + 5lbs 0:5
0% + 0lbs 0:0
And I believe it is a % of what you set in
Enter the Maximum Package Weight you will ship