我需要循环功能的特定部分

这是对先前问题的跟进，得到了一些很好的反馈，我正在学习，寄存器的工作原理和计算结果很好。我想问问是否可以仅循环一个函数的一部分，我插入了一个if语句，该语句检查该人给的钱是否少于总数，以及如果给的钱要更多。我想再次要求现金，并在收到等于或大于总额的金额后继续进行找零。这是代码：

using System.IO;
using System.Linq;
using System.Collections.Generic;

namespace CSharp_Shell
{

    public static class Program 
    {
        public static void Main() 
        {



           Console.WriteLine("                    *****--------------------*****");
           Console.WriteLine("                          Wit Groceries 2020");
           Console.WriteLine("                    *****--------------------*****");
           Console.WriteLine("            Welcome, would you like to do some shopping today?");
           Console.WriteLine("");

           var STOCK = new List<Items>
           {
            new Items{ id = 1, Name = "Bun", Price = 150},
            new Items{ id = 2, Name = "Soda", Price = 100},
            new Items{ id = 3, Name = "Cheese", Price =70},
            new Items{ id = 4, Name = "Tissue", Price = 50},
            new Items{ id = 5, Name = "Fabuloso", Price = 180},
            new Items{ id = 6, Name = "Grace Mackerel", Price = 70},
            new Items{ id = 7, Name = "Rice", Price = 50},
            new Items{ id = 8, Name = "Flour", Price = 40},
            new Items{ id = 9, Name = "Sugar", Price = 30},
           };



           var STOCKDict = STOCK.ToDictionary(i=>i.id);

           var selecteditems = new List<Items>();

           STOCK.ForEach(x => Console.WriteLine($"ID {x.id} - Name: {x.Name.PadRight(20)} Price: {string.Format("{0:C2}", x.Price)}"));
           Console.WriteLine();

           Console.WriteLine(" Enter the number for the item you want, enter 'x' when finihshed");


           do
           {
            var choice = Console.ReadLine();
            if (choice.Trim() =="x")
            {
                break;
            }
            int id;
            if (!int.TryParse(choice, out id))
            {
               Console.WriteLine("Enter number or 'x' ");
               continue;
            }
            if (STOCKDict.ContainsKey(id))
            {
                var item = STOCKDict[id];
                selecteditems.Add(item);
                Console.WriteLine(item.Name + " is added to your list");
            }
            else
            {
                Console.WriteLine(id + " is not available");
            }
           }
           while (true);

           decimal subt = selecteditems.Sum(i=>i.Price);
           decimal GCT = 16.50m;
           decimal GCTamt = (subt*GCT/100);
           decimal total = subt+GCTamt;
           Console.WriteLine("                  Sub-total = " + "$" + subt);
           Console.WriteLine("                  GCT = " + "$" + GCTamt);
           Console.WriteLine("                  You pay " + "$" +total);
           Console.WriteLine();



           Console.WriteLine("                  Please tender cash");
           decimal tend =decimal.Parse(Console.ReadLine());
            if (tend>total)
            {
           decimal change = tend - total;
           Console.WriteLine("         You gave " + " $"+tend + " Your change is " + "$"+change);
            }
            else
            {
                Console.WriteLine(" Please tender more cash");
            }



        }

    }
    public class Items
    {
        public int id {get; set;}
        public string Name{get; set;}
        public decimal Price {get;set;}
    }
}