Using TimedQueue

product.cs

namespace VendingMachines
{
    public class Product
    {
        
        public Product(string name, int coins, int available)
        {
            Name = name;
            Coins = coins;
            Available = available;
        }

        public string Name { get; }

        public int Coins { get; }

        public int Available { get;set; }
    }
}

VendingMachineBehaviour.cs

namespace VendingMachines
{
    public class VendingMachineBehaviour : MonoBehaviour
    {
        #region Serialized Fields

        [SerializeField]
        private Text _message;

        [SerializeField]
        private GameObject _productUIPrefab;

        [SerializeField]
        private Transform _productsListUI;

        [SerializeField]
        private Sprite _productAvailableTexture;

        [SerializeField]
        private Sprite _productUnavailableTexture;

        #endregion

        #region Vending Machine Data

        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;

        #endregion

        private TimedQueue<string> _messageQueue;

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

            foreach (var product in _products)
            {
                GameObject go = Instantiate(_productUIPrefab, _productsListUI);
                Transform t = go.transform;
                t.Find("Code").GetComponent<Text>().text = product.Key;
                t.Find("Name").GetComponent<Text>().text = product.Value.Name;
                t.Find("Coins").GetComponent<Text>().text = "" + product.Value.Coins;
                t.GetComponent<Image>().sprite = (product.Value.Available <= 0) ? _productUnavailableTexture : _productAvailableTexture;
            }
        }

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

        public void InsertCoin()
        {
       

            _numbers[0] = 0;
            _numbers[1] = 0;

            _coins++;

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

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

            _messageQueue.Enqueue(string.Join("", _numbers));
        }

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

            _coins = 0;
        }

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

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