I think this is your problem:
lblTotal.Text = Convert.ToInt32(
Convert.ToInt32(txt_Qty.Text)
* Convert.ToInt32(lblPrdPrice.Text)
).ToString();
tot = Convert.ToDouble(tot + lblTotal.Text);
qty = Convert.ToInt32(qty + txt_Qty.Text);
change to
lblTotal.Text = (Convert.ToInt32(txt_Qty.Text)
* Convert.ToInt32(lblPrdPrice.Text)).ToString();
tot += Convert.ToDouble(lblTotal.Text);
qty += Convert.ToInt32(txt_Qty.Text);
You have a very strange design with declaring your GUI components inside a loop like that, but who am I to judge.