Lets say you want to add 10% to the shipping cost for USA customers.
With Handling Matrix you can use the PERCENTAGE_CART option to do this.
PERCENTAGE_CART takes the price field and uses this as a percentage of the cart total. E.g. the shipping handling charge on a $10 cart would be $1. So if shipping was say $5 the total shipping and handling shown to the customer would be $6.
The example CSV file can be downloaded in the link below: