Summary of all numbers list<>

You need a collection of some form - and a List is a good idea - if you are planning of storing the entries.
But if all you are interested in is the sum or the values, then just create a simple variable to hold it, initialise it to zero, and add each value as you go round. You can then print the sum so far at any time.

But ... rather than using Convert and throwing a try ... catch in there, use int.TryParse, Doubel.TryParse or similar - it returns a boolean "success / fail" value which you can use in a if directly. Makes you code much easier to read.

To be honest, it doesn't look like you thought much before you leapt into code there: the indentation is all over the place, the spacing is weird, and I have no idea why you convert a string to an integer just to store it in a double?
Stop and think about what you are doing instead of jumping right into coding - it will save you a huge amount of time in the long run! This may help: How to Write Code to Solve a Problem, A Beginner's Guide[^]

The main idea is to work on specific data.
So, if you have a product, its price and tax rate for each product code, i'd suggest to create 2 classes:

//holds product code and tax rate for that product
public class PTax
{
	public string PCode { get; set; }
	public double TaxRate { get; set; }
}

//holds product price depending on product code 
public class PPrice
{
	public PTax PTax { get; set; }
	public double Netto { get; set; }
	public double Brutto
	{
		get  => Netto * PTax.TaxRate;
	}
}

Usage:

System.Globalization.CultureInfo ci = System.Globalization.CultureInfo.CreateSpecificCulture("pl-PL");
List<PTax> prodTax = new List<PTax>()
    {
        new PTax{PCode = "A", TaxRate = 1.00},
        new PTax{PCode = "B", TaxRate = 1.08},
        new PTax{PCode = "C", TaxRate = 1.23}
    };
List<PPrice> prodPrice = new List<PPrice>()
{
    new PPrice(){PTax = prodTax[0], Netto = 110.00},
    new PPrice(){PTax = prodTax[0], Netto = 200.00},
    new PPrice(){PTax = prodTax[0], Netto = 5.50},
    new PPrice(){PTax = prodTax[1], Netto = 500.00},
    new PPrice(){PTax = prodTax[1], Netto = 1180.15},
    new PPrice(){PTax = prodTax[2], Netto = 333.33},
    new PPrice(){PTax = prodTax[2], Netto = 2000.00}
};

Console.WriteLine("List of products and their prices:");
foreach(PPrice pp in prodPrice)
    Console.WriteLine($"Product: '{pp.PTax.PCode}'\tNet. price: {pp.Netto.ToString("C2", ci)}\tBrutto: {pp.Brutto.ToString("C2", ci)}");

var sumy = prodPrice
    .GroupBy(x => x.PTax.PCode)
    .Select(grp => new
    {
        PCode = grp.Key,
        Count = grp.Count(),
        Netto = grp.Sum(x=> x.Netto),
        Tax = grp.Select(x=> x.PTax.PCode).FirstOrDefault(),
        Brutto = grp.Sum(x=> x.Brutto)
    })
    .ToList();

Console.WriteLine("\nSum of products based on product code:");
foreach(var p in sumy)
    Console.WriteLine($"Product: '{p.PCode}'\tCount:{p.Count}\tNet. price:{p.Netto.ToString("C2", ci)}\tBrutto:{p.Brutto.ToString("C2", ci)}");

Result:

List of products and their prices:
Product: 'A'  Net. price: 110,00 zł  Brutto: 110,00 zł
Product: 'A'  Net. price: 200,00 zł  Brutto: 200,00 zł
Product: 'A'  Net. price: 5,50 zł  Brutto: 5,50 zł
Product: 'B'  Net. price: 500,00 zł  Brutto: 540,00 zł
Product: 'B'  Net. price: 1 180,15 zł  Brutto: 1 274,56 zł
Product: 'C'  Net. price: 333,33 zł  Brutto: 410,00 zł
Product: 'C'  Net. price: 2 000,00 zł  Brutto: 2 460,00 zł

Sum of products based on product code:
Product: 'A'  Count:3  Net. price:315,50 zł  Brutto:315,50 zł
Product: 'B'  Count:2  Net. price:1 680,15 zł  Brutto:1 814,56 zł
Product: 'C'  Count:2  Net. price:2 333,33 zł  Brutto:2 870,00 zł

Good luck!

What y think about it?
Now I have one problem....netto.Sum() and brutto.Sum(). Doesn't work. Can someone explain why?

<pre>using System;
using System.Collections.Generic;
using System.Linq;

namespace KasaTest
{
    class Kasa
    {
        public static void Main(string[] args)
        {
            while (true)
            {  
                Console.WriteLine("podaj kod produktu");
                try
                {
                    List<double> netto = new List<double>();
                    List<double> brutto = new List<double>();
                    string productcode = Console.ReadLine();
                    string code = productcode.Substring(0, 1);
                    double number = int.Parse(productcode.Substring(1));
                    
                    if (code == "a" || code == "A")
                    {
                        Console.WriteLine("kwota netto " + number / 100  + " kwota brutto: " + number / 100 + ",00 zł");
                        brutto.Add(number / 100);
                        netto.Add(number / 100);
                        continue;
                    }

                    if (code == "b" || code == "B")
                    {
                        Console.WriteLine("kwota netto: " + (number /100) + " kwota brutto: " + (number * 0.0108) + " zł");
                        brutto.Add(number * 0.0108);
                        netto.Add(number / 100);
                        continue;
                    }

                    else
                    { 
                        Console.WriteLine("kwota netto: " + (number / 100) + " kwota brutto: " + (number * 0.0123) + " zł");
                        brutto.Add(number * 0.0123);
                        netto.Add(number / 100);
                        continue;
                    }
                    
                       // Console.WriteLine(netto.Sum() + brutto.Sum());
                }
                catch (FormatException)
                {
                    Console.WriteLine("UWAGA! Podaj właściwy kod");
                    continue;
                }

            }
        }
    }
}

var value = 1235.58;
Console.WriteLine(value.ToString("C3", System.Globalization.CultureInfo.CreateSpecificCulture("pl-PL")));
//prints: 1 235,580 zł