Hi gmsahmod,

Refer the modified code. It will calculate each page subtotal, vat and total in respective sheet. 

public partial class Form5 : Form
{
    public Form5(DataTable dt1)
    {
        InitializeComponent();
        this.dataGridView1.DataSource = dt1;
    }

    private void Form5_Load(object sender, EventArgs e)
    {
        this.dataGridView1.Columns[0].Width = 150;
        this.dataGridView1.Columns[1].Width = 250;
        this.dataGridView1.Columns[2].Width = 40;
        this.dataGridView1.Columns[3].Width = 50;
        this.dataGridView1.Columns[4].Width = 58;
        this.dataGridView1.Columns[5].Width = 50;
        this.dataGridView1.Columns[6].Width = 50;
        this.dataGridView1.Columns[6].DefaultCellStyle.Format = "R0.00";
    }

    static string UppercaseWords(string value)
    {
        char[] a = value.ToLower().ToCharArray();

        for (int i = 0; i < a.Length; i++)
        {
            a[i] = i == 0 || a[i - 1] == ' ' ? char.ToUpper(a[i]) : a[i];

        }

        return new string(a);
    }

    private void button1_Click(object sender, EventArgs e)
    {
        Excel.Application xlApp;
        Excel.Workbook xlWorkBook;
        Excel.Worksheet xlWorkSheet;

        object misValue = System.Reflection.Missing.Value;
        xlApp = new Excel.Application();
        string filePath = @"C:\\masterquote.xlsx";
        xlWorkBook = xlApp.Workbooks.Add(filePath);

        string datestr = DateTime.Now.ToShortDateString();
        string datestr1 = DateTime.Now.ToShortTimeString();
        string fileName = @"C:\Quote_" + datestr.Replace('/', '_') + " " + datestr1.Replace(':', '_') + ".xlsx";
        
        string quote = datestr.Replace('/', '_') + " " + datestr1.Replace(':', '_');
        string salesman = "test@gmail.com";
        string customer = "test";
        string contact = "test contact";
        //string delivery = txtDel.Text.ToUpper();
        string address2 = "testAddress2".ToUpper();
        string address3 = "testAddress3".ToUpper();
        string note = "note".ToUpper();
        string trans = "tran 1";

        int currentSheet = 1;
        int cell = 0;
        int rowsToDisplay = 28;
        int rowsToStart = 20;
        for (int row = 0; row < dataGridView1.Rows.Count; row++)
        {
            if (currentSheet <= xlWorkBook.Sheets.Count)
            {
                xlWorkSheet = (Excel.Worksheet)xlWorkBook.Worksheets.get_Item(currentSheet);
            }
            else
            {
                var xlSheets = xlWorkBook.Sheets as Excel.Sheets;
                xlWorkSheet = (Excel.Worksheet)xlSheets.Add(misValue, xlSheets[xlWorkBook.Sheets.Count], misValue, misValue);
                xlWorkSheet.Name = "Sheet" + currentSheet.ToString();
            }
            xlWorkSheet.get_Range("E11", "E11").Value2 = salesman;
            xlWorkSheet.get_Range("A12", "A12").Value2 = customer;
            xlWorkSheet.get_Range("A13", "A13").Value2 = address2;
            xlWorkSheet.get_Range("A14", "A14").Value2 = address3;
            xlWorkSheet.get_Range("B11", "B11").Value2 = contact;
            xlWorkSheet.get_Range("D12", "D12").Font.Bold = true;
            xlWorkSheet.get_Range("B17", "B17").Value2 = DateTime.Now;
            xlWorkSheet.get_Range("E17", "G17").Value2 = quote;
            xlWorkSheet.get_Range("E13", "H13").Value2 = note;
            xlWorkSheet.get_Range("C17", "C17").Value2 = trans;

            for (int column = 0; column < dataGridView1.Columns.Count; column++)
            {
                if (dataGridView1.Rows[row].Cells[column].Value == null)
                {
                    xlWorkSheet.Cells[cell + rowsToStart, column + 1] = "";
                }
                else
                {
                    xlWorkSheet.Cells[cell + rowsToStart, column + 1] = dataGridView1.Rows[row].Cells[column].Value.ToString();
                }
            }

            if ((row + 1) % rowsToDisplay == 0)
            {
                xlWorkSheet.get_Range("F49", "F49").Value2 = "total";
                xlWorkSheet.get_Range("G49", "G49").Value2 = "=SUM(G20:G48)/1.15";

                xlWorkSheet.get_Range("F50", "F50").Value2 = "Vat @15%";
                xlWorkSheet.get_Range("G50", "G50").Value2 = "=SUM(F20:F48)+(SUM(F20:F48)*15%)";

                xlWorkSheet.get_Range("F51", "F51").Value2 = "Total";
                xlWorkSheet.get_Range("G51", "G51").Value2 = "=SUM(G49:G50)";
                currentSheet++;
                cell = 0;
            }
            else if (row + 1 == dataGridView1.Rows.Count)
            {
                xlWorkSheet.get_Range("F49", "F49").Value2 = "total";
                xlWorkSheet.get_Range("G49", "G49").Value2 = "=SUM(G20:G48)/1.15";

                xlWorkSheet.get_Range("F50", "F50").Value2 = "Vat @15%";
                xlWorkSheet.get_Range("G50", "G50").Value2 = "=SUM(F20:F48)+(SUM(F20:F48)*15%)";

                xlWorkSheet.get_Range("F51", "F51").Value2 = "Total";
                xlWorkSheet.get_Range("G51", "G51").Value2 = "=SUM(G49:G50)";
            }
            else
            {
                cell++;
            }
        }
        if (System.IO.File.Exists(fileName))
        {
            System.IO.File.Delete(fileName);
        }

        xlWorkBook.SaveAs(fileName, Excel.XlFileFormat.xlOpenXMLWorkbook, misValue, misValue, misValue, misValue, Excel.XlSaveAsAccessMode.xlExclusive, misValue, misValue, misValue, misValue, misValue);
        xlWorkBook.Close(true, misValue, misValue);
        xlApp.Quit();

        //releaseObject(xlWorkSheet);
        releaseObject(xlWorkBook);
        releaseObject(xlApp);
        MessageBox.Show("Excel file created , you can find the file c:\\MyQuotes" + fileName);
    }

    private void releaseObject(object obj)
    {
        try
        {
            System.Runtime.InteropServices.Marshal.ReleaseComObject(obj);
            obj = null;
        }
        catch (Exception ex)
        {
            obj = null;
            MessageBox.Show("Exception Occured while releasing object " + ex.ToString());
        }
        finally
        {
            GC.Collect();
        }
    }
}