You can calculate your shipping prices by a product's weight by using shipping units. Shipping units can be set product by product and act as a multiplier against the unit cost of that product. So, to price your shipping by kg, set the shipping units of each product to its weight in kg and then set the per unit price in the shipping area to be the price per kg.
Setting a Product's Shipping Units
- Navigate to the product in edit mode.
- Choose Edit Stock & Pricing from the THIS PRODUCT menu.
- Move to the shipping charges step.
- Enter the shipping units. This can be a decimal number such as 1.8
- Click FINISH.