Compute Subtotal of POS

Question

So I'm creating a project, which is a Point of Sale, like those in fast food chains.

The buttons on my POS is created dynamically, depends on the values from my database, and now I'm having a hard time to compute the subtotal when I change the quantity of each item. I used DataGrid to list all the products ordered by the customer.

I created two buttons which is add and minus that can set the quantity of the selected row in the datagridview, I'm not sure if I got it right but the code is also provided below which computes the price of the selected item multiplied to the quantity.

My problem is, how can I compute the subtotal price, and the total quantity of items in my datagridview everytime I add items in my datagrid or I add or subtract in the quantity of the item.? The subtotal should reflect immediately EVERYTIME I add an item, or add or subtract an item.

Provided is a sample image to understand better what I want to happen in my project.

public void quantity_change(object sender, EventArgs e)
    {
        var row = dataGridView1.CurrentRow;

        if (row == null || row.Index < 0)
            return;
        var unit = (sender == add) ? 1 : -1;

        var quantity = Convert.ToInt32(row.Cells["Quantity"].Value) + unit;

        row.Cells["Quantity"].Value = quantity;
        var rate = Convert.ToDouble(row.Cells["SellingPrice"].Value);
        row.Cells["TotalPrice"].Value = quantity * rate;
    }

private void frmPOS_Load(object sender, EventArgs e)
    {

        dataGridView1.SelectionMode = DataGridViewSelectionMode.FullRowSelect;

        add.Click += quantity_change;
        minus.Click += quantity_change;

        cmd = new MySqlCommand("SELECT * FROM tblmenu", dbConn);
        MySqlDataReader rdr = cmd.ExecuteReader();

        while (rdr.Read())
        {
            Button btn = new Button();
            btn.Text = rdr["menuName"].ToString();
            btn.Name = rdr["menuID"].ToString();
            btn.Width = 126;
            btn.Height = 80;

            btn.Click += delegate
            {
                dataGridView1.ClearSelection();

                MySqlConnection cnn2 = new MySqlConnection(sqlConn.connString);
                cnn2.Open();
                cmd = new MySqlCommand("SELECT menuName, menuPrice FROM tblmenu WHERE menuID = @id", cnn2);
                cmd.Parameters.AddWithValue("@id", btn.Name);
                MySqlDataReader rdr2 = cmd.ExecuteReader();

                while (rdr2.Read())
                {
                   //I added the item in my datagridview, with the button name, 1 = 1quantity, and Selling Price
                 dataGridView1.Rows.Add(rdr2.GetString("menuName").ToUpper(), 1, rdr2.GetDouble("menuPrice"));
                }

                //I copied the value of Selling Price Column to the Total Price Column in this part
                foreach (DataGridViewRow row in dataGridView1.Rows)
                {
                    value = row.Cells["SellingPrice"].Value.ToString();
                    row.Cells["TotalPrice"].Value = value;
                }
            };


            if (rdr["menuAvailability"].ToString() == "yes")
            {
                if (rdr["menuCategory"].ToString() == "Sandwiches")
                {
                    flpSandwiches.Controls.Add(btn);
                }
                else if (rdr["menuCategory"].ToString() == "Appetizers")
                {
                    flpAppetizers.Controls.Add(btn);
                }
            }
        }

        rdr.Close();
    }