Shipment rule calculates weight of an order/article and adds to customer bills automatically during ordering. What weights you set in the articles help this process. Follow below steps to set the weight of all your articles that can be subjected adding extra shipping costs.
1. Open "Admin > Customize > Content" and click on "Click to edit your app"
2. Click on an article to add the weight
3. Click on "More Options"
4. Click on "Weight" and enter the weight of the article