Here the client wanted to add a 15% handling charge based on cart subtotal for USA customers, and 30% for Rest of World.  The Handling Matrix extension does that very easily using a setup as shown in attached CSV.

Add Extra Weight to the Cart If Certain Items are Present

You can do this with the "WEIGHTINC=amount of weight to add" rule. Take a look at the CSV file for how to do this.  (As of zip version 5.2)

Add a Handling Fee to Specific Delivery Methods

You can add a handling fee to the cart for specific delivery methods (as of zip version 5.2). So say you get charged a customs charge when shipping to Canada via Air, you could add on $20 for air shipments to cover this cost. Take a look below for how to do this.

N.B: The delivery type text in the CSV file MUST match what is shown on the front end exactly. This includes spaces and capitalisation.

N.B: The delivery type text in the CSV file MUST match what is shown on the front end exactly. This includes spaces and capitalisation.

Decrease Shipping Charges by a Percentage based on Cart Subtotal

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Let's say you want to give a 20% discount when the cart subtotal is over 100USD. With Handling Matrix you can do this. You can even vary to give a 40% discount when over 500USD.   Use the PERCENTAGE_SHIP algorithm to implement.

Increase Shipping by 50% on 2nd Day Air for some Products