The default shipping charge can be changed in the vendors settings page (Certificates > Vendors). Edit the desired vendor and change the default shipping amount. This is the default amount that is calculated when a new purchase order is created.
Each purchase order has the shipping included based on the vendor settings. You may change the shipping for each purchase order individually by clicking on the shipping amount in purchase order. After clicking on the shipping value, the option to change the shipping charge will appear.
Note: To reset the shipping back to the default shipping click the reset button.