feat: show live estimated eBay Final Value Fee (12.8%) below price field

EbayListingTool/Views/NewListingView.xaml

@@ -435,16 +435,21 @@
                                     </StackPanel>
                                 </Button>
                             </Grid>
-                            <mah:NumericUpDown x:Name="BPriceBox"
+                            <mah:NumericUpDown x:Name="BPriceBox" ValueChanged="PriceBox_ValueChanged"
                                                StringFormat="&#x00A3;{0:0.00}"
                                                Minimum="0" Maximum="99999"
                                                Interval="0.50"
                                                Margin="0,0,0,4"
                                                AutomationProperties.LabeledBy="{Binding ElementName=PriceLabel}"/>
                             <TextBlock x:Name="BPriceHint"
-                                       FontSize="12" Margin="0,0,0,12"
+                                       FontSize="12" Margin="0,0,0,4"
                                        Visibility="Collapsed"
                                        Foreground="{DynamicResource MahApps.Brushes.Gray5}"/>
+                            <TextBlock x:Name="BFeeLabel"
+                                       FontSize="12" Margin="0,0,0,12"
+                                       Visibility="Collapsed"
+                                       Foreground="{DynamicResource MahApps.Brushes.Gray5}"
+                                       AutomationProperties.Name="Estimated eBay listing fee"/>
 
                             <!-- Postage + Postcode -->
                             <Grid Margin="0,12,0,0">
@@ -457,7 +462,7 @@
                                     <TextBlock x:Name="PostageLabel" Text="Postage"
                                                Style="{StaticResource FieldLabel}"
                                                Margin="0,0,0,4"/>
-                                    <ComboBox x:Name="BPostageBox"
+                                    <ComboBox x:Name="BPostageBox" SelectionChanged="PostageBox_SelectionChanged"
                                               AutomationProperties.LabeledBy="{Binding ElementName=PostageLabel}">
                                         <ComboBoxItem Content="Royal Mail 1st Class"  Tag="RoyalMailFirstClass"/>
                                         <ComboBoxItem Content="Royal Mail 2nd Class"  Tag="RoyalMailSecondClass" IsSelected="True"/>

EbayListingTool/Views/NewListingView.xaml.cs

@@ -428,6 +428,39 @@ public partial class NewListingView : UserControl
         finally { SetPriceBusy(false); }
     }
 
+    private void PriceBox_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double?> e)
+        => UpdateFeeEstimate();
+
+    private void PostageBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
+        => UpdateFeeEstimate();
+
+    private static readonly Dictionary<string, decimal> PostageEstimates = new()
+    {
+        ["RoyalMailFirstClass"]  = 3.70m,
+        ["RoyalMailSecondClass"] = 2.85m,
+        ["RoyalMailTracked24"]   = 4.35m,
+        ["RoyalMailTracked48"]   = 3.60m,
+        ["CollectionOnly"]       = 0m,
+        ["FreePostage"]          = 0m,
+    };
+
+    private void UpdateFeeEstimate()
+    {
+        if (BFeeLabel == null) return;
+        var price = (decimal)(BPriceBox?.Value ?? 0);
+        if (price <= 0) { BFeeLabel.Visibility = Visibility.Collapsed; return; }
+
+        var postageTag = (BPostageBox?.SelectedItem as ComboBoxItem)?.Tag?.ToString() ?? "";
+        PostageEstimates.TryGetValue(postageTag, out var postageEst);
+
+        const decimal fvfRate = 0.128m;
+        const decimal minFee  = 0.30m;
+        var fee = Math.Max(Math.Round((price + postageEst) * fvfRate, 2), minFee);
+
+        var postageNote = postageEst > 0 ? $" + est. \u00A3{postageEst:F2} postage" : "";
+        BFeeLabel.Text       = $"Est. eBay fee: \u00A3{fee:F2}  (12.8% of \u00A3{price:F2}{postageNote})";
+        BFeeLabel.Visibility = Visibility.Visible;
+    }
     private void SetPriceBusy(bool busy)
     {
         AiPriceBtn.IsEnabled    = !busy;