def sum_columns(df, columns):
    """Get the sum of a list of columns.

        Args:
            df: Pandas DataFrame.
            columns: List of columns to sum.

        Returns:
            Value of summed columns.

        Usage:
            columns = ['quote_includes_sample', 'quote_free_shipping']
            df['sum'] = sum_columns(df, columns)
        """
    for col in columns:
        df[col] = df[col].astype(int)

    total = df[columns].astype(int).sum(axis=1)
    return total