One shipping cost for the first item and another one for each additional item

1. Log in your administrator panel and open the Shipping methods page (the link to this page is in the SHIPPINGS/TAXES sidebox on the left of the page).
2. Create a new shipping method or choose the existing one.
3. Click on the Edit button of this shipping method. The Shipping method properties page will be open.
4. On that page select the desired destination in the Show rates for destination: box and click on its link.
5. In the Cost dependences section enter 0 into the More than $ input field and price for the first item (e.g. 4.50) into the Rate value input field.
6. In the Items dependences section enter 0 into the More than.. items input field and additional charge for the first and each additional item (e.g. 1.50) into the Rate value input field. Select the Per item check box in the row.
7. Click on the Update button at the bottom to save the changes.
   
   The shipping method is set up. The shipping cost will be $6 if one item is ordered and extra $1.50 will be added to the cost per each additional item in case of 2 or more items.
   

   NOTE: This extra charge does not depend on the ordering products. For example, if a customer adds the third product to the card and it is different from the first two, additional charge for it will be $1.50 anyway.
   

Also there is an ability to define additional shipping freight for a product which will be added to the calculated shipping cost for each ordered item:

1. Click on the Manage products link in the CATALOG side box on the left of the page.
2. Choose the product for which the additional cost will be added to shipping cost for each additional product.
3. On the Update products page enter cost (e.g. 2.00) which will be added to the shipping cost for each additional product in the Shipping freight input field.

   NOTE: It is necessary to fill in the input field for all products that will have such additional shipping charges.
   

4. Click on the Save changes button.
   

   As a result of defining the shipping freight, the shipping cost will be $8 ($6 + $2) if one item is ordered and $3.50 ($1.50 + $2.00) will be added to the cost per each additional item in case of 2 or more items.