basically, one of your strings isn't an int string.
To work out which one, try separating out your code:
for (int i = 0; i < dgv_products_view.Rows.Count-1; i++)
{
var cell0 = Convert.ToInt32(dgv_products_view.Rows[i].Cells[0].Value);
var orderId = Convert.ToInt32(txt_order_id.Text);
var cell3 = Convert.ToInt32(dgv_products_view.Rows[i].Cells[3].Value);
var cell5 = float.Parse(dgv_products_view.Rows[i].Cells[5].Value.ToString());
order.ADD_ORDER_DETAILS(cell0,orderId,cell3,
dgv_products_view.Rows[i].Cells[2].Value.ToString(),
cell5,
dgv_products_view.Rows[i].Cells[4].Value.ToString(),
dgv_products_view.Rows[i].Cells[6].Value.ToString());
}
hope that helps ^_^