How to handle the EXCEPTION INPUT STRING NOT in CORRECT FORMAT

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C#

in my code when i check my checkboxes the textbox beside it enables so when i add the values together in the textboxes using an array it keeps on giving me an exception warning saying input string format not correct


What I have tried:

const double Price_Friedrice = 200;
        const double Price_Jollofrice = 200;
        const double Price_Spags = 200;
        const double Price_Beans = 200;
        const double Price_Fries = 600;
        const double Price_Salad = 100;
        const double Price_Chicken= 300;
        const double Price_Plantain = 50;
        const double Price_Egg = 50;
        const double Price_Fish = 100;
        const double Price_Cocorice = 300;
        const double Price_Turkey = 500;
        const double Price_Moin = 50;
        const double Price_breadandegg = 200;
        const double Price_ewagee = 200;
        const double Price_Whiterice = 200;
        const double Price_Ofada = 200;
        const double Price_Gizzard = 50;
        const double Price_Bole = 500;
        const double Price_Burritos = 500;
        const double Price_Smallchops = 500;
        const double Price_Pizza = 600;
        const double Price_Sharwama = 600;
        const double Price_Suya = 400;
        const double Price_Waffles = 500;
        const double Price_Water = 50;
        const double Price_Exotic = 400;
        const double Price_Coke = 120;
        const double Price_Fanta = 120;
        const double Price_Pepsi = 120;
        const double Price_Mirinda = 100;
        const double Price_Slush = 200;
        const double Price_Juice = 250;
        const double Price_Sobo = 100;
        const double Price_Eba = 100;
        const double Price_Poundoyam = 100;
        const double Price_Semo = 50;
        const double Price_Egusi = 0;
        const double Price_Vegetable = 0;
        const double Price_Ewedu = 0;
        const double Price_Okra = 0;
        const double Price_WhiteSoup = 0;
        const double Tax_rate = 20;

        double  iTax, iSubtotal, iTotal;

private void Total_btn_Click(object sender, EventArgs e)
       {
           double[] cost_of_Item = new double[41];
           cost_of_Item[0] = Convert.ToDouble(friedricetxt.Text) * Price_Friedrice;
           cost_of_Item[1] = Convert.ToDouble(jolloftxt.Text) * Price_Jollofrice;
           cost_of_Item[2] = Convert.ToDouble(spagstxt.Text) * Price_Spags;
           cost_of_Item[3] = Convert.ToDouble(Beanstxt.Text) * Price_Beans;
           cost_of_Item[4] = Convert.ToDouble(Fries_txt.Text) * Price_Fries;
           cost_of_Item[5] = Convert.ToDouble(salad_txt.Text) * Price_Salad;
           cost_of_Item[6] = Convert.ToDouble(chicken_txt.Text) * Price_Chicken;
           cost_of_Item[7] = Convert.ToDouble(plantain_txt.Text) * Price_Plantain;
           cost_of_Item[8] = Convert.ToDouble(Egg_txt.Text) * Price_Egg;
           cost_of_Item[9] = Convert.ToDouble(fish_txt.Text) * Price_Fish;
           cost_of_Item[10] = Convert.ToDouble(Cocorice_txt.Text) * Price_Cocorice;
           cost_of_Item[11] = Convert.ToDouble(Turkey_txt.Text) * Price_Turkey;
           cost_of_Item[12] = Convert.ToDouble(moin_txt.Text) * Price_Moin;
           cost_of_Item[13] = Convert.ToDouble(Breadegg_txt.Text) * Price_breadandegg;
           cost_of_Item[14] = Convert.ToDouble(whiterice_txt.Text) * Price_Whiterice;
           cost_of_Item[15] = Convert.ToDouble(Ofada_txt.Text) * Price_Ofada;
           cost_of_Item[16] = Convert.ToDouble(Gizzard_txt.Text) * Price_Gizzard;
           cost_of_Item[17] = Convert.ToDouble(Bole_txt.Text) * Price_Bole;
           cost_of_Item[18] = Convert.ToDouble(Burritos_txt.Text) * Price_Burritos;
           cost_of_Item[19] = Convert.ToDouble(Smallchops_txt.Text) * Price_Smallchops;
           cost_of_Item[20] = Convert.ToDouble(pizza_txt.Text) * Price_Pizza;
           cost_of_Item[21] = Convert.ToDouble(Sharwama_txt.Text) * Price_Sharwama;
           cost_of_Item[22] = Convert.ToDouble(Suya_txt.Text) * Price_Suya;
           cost_of_Item[23] = Convert.ToDouble(Waffles_txt.Text) * Price_Waffles;
           cost_of_Item[24] = Convert.ToDouble(Water_txt.Text) * Price_Water;
           cost_of_Item[25] = Convert.ToDouble(Exotic_txt.Text) * Price_Exotic;
           cost_of_Item[26] = Convert.ToDouble(Coke_txt.Text) * Price_Coke;
           cost_of_Item[27] = Convert.ToDouble(Fanta_txt.Text) * Price_Fanta;
           cost_of_Item[28] = Convert.ToDouble(Pepsi_txt.Text) * Price_Pepsi;
           cost_of_Item[29] = Convert.ToDouble(Mirinda_txt.Text) * Price_Mirinda;
           cost_of_Item[30] = Convert.ToDouble(Slush_txt.Text) * Price_Slush;
           cost_of_Item[31] = Convert.ToDouble(Fuitjuicetxt.Text) * Price_Juice;
           cost_of_Item[32] = Convert.ToDouble(Sobo_txt.Text) * Price_Sobo;
           cost_of_Item[33] = Convert.ToDouble(eba_txt.Text) * Price_Eba;
           cost_of_Item[34] = Convert.ToDouble(poundo_txt.Text) * Price_Poundoyam;
           cost_of_Item[35] = Convert.ToDouble(Semo_txt.Text) * Price_Semo;
           cost_of_Item[36] = Convert.ToDouble(egusi_txt.Text) * Price_Egusi;
           cost_of_Item[37] = Convert.ToDouble(Vegetable_txt.Text) * Price_Vegetable;
           cost_of_Item[38] = Convert.ToDouble(Ewedu_txt.Text) * Price_Ewedu;
           cost_of_Item[39] = Convert.ToDouble(Okra_txt.Text) * Price_Okra;
           cost_of_Item[40] = Convert.ToDouble(Whitesoup_txt.Text) * Price_WhiteSoup;

           double cost, Change;
           if (method_payment.Text == "Cash")
           {
               iSubtotal = cost_of_Item[0] + cost_of_Item[1] + cost_of_Item[2] + cost_of_Item[3] + cost_of_Item[4] + cost_of_Item[5] +
               cost_of_Item[6] + cost_of_Item[7] + cost_of_Item[8] + cost_of_Item[9] + cost_of_Item[10] + cost_of_Item[11] + cost_of_Item[12]
               + cost_of_Item[13] + cost_of_Item[14] + cost_of_Item[15] + cost_of_Item[16] + cost_of_Item[17] + cost_of_Item[18] + cost_of_Item[19]
               + cost_of_Item[20] + cost_of_Item[21] + cost_of_Item[22] + cost_of_Item[23] + cost_of_Item[24] + cost_of_Item[25] + cost_of_Item[26]
               + cost_of_Item[27] + cost_of_Item[28] + cost_of_Item[29] + cost_of_Item[30] + cost_of_Item[31] + cost_of_Item[32] + cost_of_Item[33]
               + cost_of_Item[34] + cost_of_Item[35] + cost_of_Item[36] + cost_of_Item[37] + cost_of_Item[38] + cost_of_Item[39] + cost_of_Item[40];

               Subtotlbl.Text = Convert.ToString(iSubtotal);
               iTax = (iSubtotal * Tax_rate)/100;
               Taxlbl.Text = Convert.ToString(iTax);
               iTotal = (iSubtotal + iTax);
               Totallbl.Text = Convert.ToString(iTotal);
               Change = Convert.ToDouble(method_payment.Text);
               cost = Change - (iTotal);