VendingMachine.cs

 public class VendingMachine
    {
        private Dictionary<string, Product> _products = new Dictionary<string, Product>()
        {
            { "01", new Product("Product 01", 3, 5)},
            { "02", new Product("Product 02", 2, 1)},
            { "03", new Product("Product 03", 1, 0)},
            { "11", new Product("Product 04", 4, 0)},
            { "12", new Product("Product 05", 1, 5)},
            { "13", new Product("Product 06", 1, 5)},
            { "21", new Product("Product 07", 2, 3)},
            { "22", new Product("Product 08", 2, 2)},
            { "23", new Product("Product 09", 3, 5)},
            { "31", new Product("Product 10", 4, 0)},
            { "32", new Product("Product 11", 2, 1)},
            { "33", new Product("Product 12", 1, 1)},
        };

        private int[] _numbers = new[] { 0, 0 };

        private int _coins = 0;

        public Dictionary<string, Product> Products => _products;

        public int[] Numbers => _numbers;

        public int Coins => _coins;


        public void InsertCoin()
        {
            _numbers[0] = 0;
            _numbers[1] = 0;

            _coins++;
        }

        public void EnterNumber(int number)
        {
            _numbers[0] = _numbers[1];
            _numbers[1] = number;
        }

        public void Select()
        {
            if (_products.TryGetValue(string.Join("", _numbers), out Product product))
            {

                if (product.Available > 0)
                {

                    if (product.Coins <= _coins)
                    {


                        _coins -= product.Coins;

                        product.Available--;

                        //_messageQueue.Enqueue("Selling " + product.Name);

                        //_messageQueue.Enqueue("Returning " + _coins + " coins");

                        _coins = 0;
                    }
                    else
                    {
                        //_messageQueue.Enqueue("Need " + product.Coins + " coins for " + product.Name);
                    }
                }
                else
                {
                    //_messageQueue.Enqueue("Sorry, " + product.Name + "sold out");
                }
            }
            else
            {
                //_messageQueue.Enqueue("Invalid choice");
            }
        }

        public void Cancel()
        {
            _coins = 0;
            //_messageQueue.Enqueue("Returning " + _coins + " coins");
        }
    }

VendingMachineBehaviour.cs

public class VendingMachineView : MonoBehaviour
    {
        #region Serialized Fields

        [SerializeField]
        private Text _message;

        [SerializeField]
        private ProductView _productUIPrefab;

        [SerializeField]
        private Transform _productsListUI;
        #endregion

        private TimedQueue<string> _messageQueue;
        private VendingMachine _vendingMachine;

        void Start()
        {
            _messageQueue = new TimedQueue<string>(ShowMessage, ShowDefaultMessage);

            _vendingMachine = new VendingMachine();

            foreach (var product in _vendingMachine.Products)
            { 
                ProductView view = _productUIPrefab.Create(_productsListUI);
                view.Code = product.Key;
                view.Product = product.Value;
            }
        }

        void Update()
        {
            _messageQueue.Update(Time.deltaTime);
        }

          public void OnCoinButtonClicked()
        {
            _vendingMachine.InsertCoin();

            _messageQueue.Enqueue("You entered " + _vendingMachine.Coins + " coins");
        }

        public void OnNumberButtonClicked(int number)
        {
            _vendingMachine.EnterNumber(number);

            _messageQueue.Enqueue(string.Join("", _vendingMachine.Numbers));
        }

        public void OnSelectButtonClicked()
        {
            _vendingMachine.Select();
        }

        public void OnCancelButtonClicked()
        { 
            _vendingMachine.Cancel();
        }

        private void ShowMessage(string message)
        {
            _message.text = message;
        }

        private void ShowDefaultMessage()
        {
            _message.text = "Select a product";
        }
    }