Firstly, you need to keep TotalPrice not as a string, but as a number of some form: int, float, decimal - one of those is probably what you want. You can then add the new ordered item price into it, and set the TextBox appropriately:
txtTotalPrice.Text = TotalPrice.ToString();
Secondly, you need to explain in better detail what you are trying to do - I can't work out from that description what you are trying to achieve.