To make a shipping cost depend on the number of products:
  1. In the administration panel, go to Administration > Shipping & Taxes > Shipping methods.
  2. Create a new shipping method or choose the existing one.
  3. Click on the name of the shipping method. The Editing shipping method page will open.
  4. Open the Shipping charges tab and select the necessary location in the Show rates for location box.
  5. In the Items dependencies section, enter the number of product items in the More than input field and specify the shipping rate value. Finally, select the shipping rate type (absolute or percentage value).
  6. To add more rates, click on + and repeat the previous step.
  7. Click the Create button.