With the code in yellow, we define the FooterTemplate of the "Freight" column. With the code in green, we define the "sum" aggregate for the "Freight" column.
In the attached example, the "sum" displayed in the footer template is the sum of all fields in the "Freight" column. If you want to display only the sum of the fields that are loaded in the current Grid page, you can check this SO thread.
I hope the provided example will help you implement the targeted functionality in the application you are working on.
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