Получите значение в поле ввода, затем отразите/реагируйте на метку в форме xamarin

Исходя из сложности вашего кода, я рекомендую использовать для реализации шаблон MVVM. Я создал демо и выполнил вашу функцию.

Вы можете обратиться к следующему коду:

1. создайте модель представления MyViewModel.cs

public class MyViewModel: INotifyPropertyChanged 
    {
        public ObservableCollection<Delivery> Deliveries { get; set; }

        private Delivery _deliverySelectedItem;
        public Delivery DeliverySelectedItem
        {
            get => _deliverySelectedItem;
            set { 

                SetProperty(ref _deliverySelectedItem, value);

                // update the TotalAmount
                caculateTotalAmount();

            }
        }

        public ObservableCollection<Product> Products { get; set; }
        //add SelectedItem here
        private Product _productSelectedItem;
        public Product ProductSelectedItem
        {
            get => _productSelectedItem;
            set {
                
                SetProperty(ref _productSelectedItem, value);

                // update the TotalAmount
                caculateTotalAmount();

            }
        }


        private int _quantity;
        public int Quantity
        {
            get => _quantity;
            set
            {

                SetProperty(ref _quantity, value);

                // update the TotalAmount
                caculateTotalAmount();
            }
        }

        private int _totalAmount;
        public int TotalAmount
        {
            get => _totalAmount;
            set
            {

                SetProperty(ref _totalAmount, value);
            }
        }


        private void caculateTotalAmount() {

            if (String.IsNullOrEmpty(Quantity.ToString() )  || Quantity == 0) {
                TotalAmount = 0;

                return;
            }

            if (ProductSelectedItem!=null  && DeliverySelectedItem!=null) {

                TotalAmount = ProductSelectedItem.productPrice * Quantity + DeliverySelectedItem.deliveryFee;
            }
        }


        public MyViewModel() {

            Products = new ObservableCollection<Product>();
            Products.Add(new Product { ProductId = 01, productType = "Products", productPrice = 10 });
            Products.Add(new Product { ProductId = 02, productType = "02", productPrice = 12 });
            Products.Add(new Product { ProductId = 03, productType = "Products", productPrice = 13 });
            Products.Add(new Product { ProductId = 04, productType = "Products", productPrice = 15 });


            Deliveries = new ObservableCollection<Delivery>();
            Deliveries.Add(new Delivery {  deliveryFee = 10, deliveryType = "Express" });
            Deliveries.Add(new Delivery { deliveryFee = 20, deliveryType = "Standard" });
            Deliveries.Add(new Delivery { deliveryFee = 30, deliveryType = "Standard" });


        }

        bool SetProperty<T>(ref T storage, T value, [CallerMemberName] string propertyName = null)
        {
            if (Object.Equals(storage, value))
                return false;

            storage = value;
            OnPropertyChanged(propertyName);
            return true;
        }

        protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        public event PropertyChangedEventHandler PropertyChanged;

    }

2.создать класс Delivery.cs и Product.cs

public class Delivery 
{
    public string deliveryType { get; set; }

    public int deliveryFee { get; set;}
}

public class Product 
{
    public int ProductId { get; set; }
    public string productType { get; set; }
    public int productPrice { get; set; }
}

3.MainPage.xaml

<?xml version = "1.0" encoding = "utf-8" ?> 
<ContentPage xmlns = "http://xamarin.com/schemas/2014/forms"
             xmlns:x = "http://schemas.microsoft.com/winfx/2009/xaml" 
             xmlns:pickerapp2023112 = "clr-namespace:PickerApp2023112"
             x:Class = "PickerApp2023112.MainPage">

    <ContentPage.BindingContext>
        <pickerapp2023112:MyViewModel></pickerapp2023112:MyViewModel>
    </ContentPage.BindingContext>

    <StackLayout>
        <Picker x:Name = "Picker_DeliveryType"  ItemsSource = "{Binding Deliveries}" ItemDisplayBinding = "{Binding deliveryFee}"  SelectedItem = "{Binding DeliverySelectedItem}"
        Title = "Select a delivery type"
        TitleColor = "Red">

        </Picker>

        <Picker x:Name = "Picker_ProductType"  ItemsSource = "{Binding Products}" ItemDisplayBinding = "{Binding productPrice}" SelectedItem = "{Binding ProductSelectedItem}"
        Title = "Select a product type"
        TitleColor = "Red">

        </Picker>

        <StackLayout  Orientation = "Horizontal">
            <Label  Text = "Please input quantity: "  BackgroundColor = "CadetBlue"/>

            <Entry Placeholder = "0"  Text = "{Binding Quantity}" TextColor = "Red" HorizontalOptions = "FillAndExpand"></Entry>

        </StackLayout>



        <Label  x:Name = "labelDeliveryFee" Text = "{Binding DeliverySelectedItem.deliveryFee,StringFormat='The delivery free is {0:F1}'}"    HorizontalOptions = "StartAndExpand" BackgroundColor = "Yellow"></Label>

        <Label  x:Name = "labelProductPrice" Text = "{Binding ProductSelectedItem.productPrice,StringFormat='The product price is {0:F2}'}" HorizontalOptions = "StartAndExpand" BackgroundColor = "Yellow"></Label>

        <StackLayout  Orientation = "Horizontal">
            <Label  Text = "The total amount: "  BackgroundColor = "CadetBlue"/>

            <Entry Placeholder = "0"  Text = "{Binding TotalAmount}" TextColor = "Red" HorizontalOptions = "FillAndExpand"></Entry>

        </StackLayout>

    </StackLayout>

</ContentPage>

Примечание:

1. Я добавляю два объекта для свойства SelectedItem двух средств выбора и реализую интерфейс INotifyPropertyChanged для этой ViewModel, если мы изменим значение свойства, пользовательский интерфейс будет обновляться автоматически. То же самое верно и для других свойств.

    private Delivery _deliverySelectedItem;
    public Delivery DeliverySelectedItem
    {
        get => _deliverySelectedItem;
        set { 

            SetProperty(ref _deliverySelectedItem, value);
        }
    }

    public ObservableCollection<Product> Products { get; set; }
    //add SelectedItem here
    private Product _productSelectedItem;
    public Product ProductSelectedItem
    {
        get => _productSelectedItem;
        set {
            
            SetProperty(ref _productSelectedItem, value);

        }
    }

В этом состоянии нам не нужно добавлять событие SelectedIndexChanged для Picker.