try this
decimal num =0,num2=0,num3=0,num4=0,num5=0,num6=0;
decimal.TryParse(txtcustomot.Text,out num);
decimal.TryParse(txtAirfreight.Text,out num2);
decimal.TryParse(txtDocument.Text,out num3);
decimal.TryParse(txtcompt.Text,out num4);
decimal.TryParse(txtairlanka.Text,out num5);
decimal.TryParse(txtcexp.Text,out num6);
decimal num8 = num + num2 + num3 + num4 + num5 + num6;
lbltotal0.Text = num8.ToString();