To calculate the sum of a column in a DataTable use the DataTable.Compute method.
Example of usage from the linked MSDN article:
DataTable table = dataSet.Tables["YourTableName"];
// Declare an object variable.
object sumObject;
sumObject = table.Compute("Sum(Amount)", string.Empty);
Display the result in your Total Amount Label like so:
lblTotalAmount.Text = sumObject.ToString();