[#48] Billing: Add custom modifiers for members

Elwig/Windows/MailWindow.xaml.cs

@@ -535,6 +535,7 @@ namespace Elwig.Windows {
                                     data.Item3.ConsiderContractPenalties,
                                     data.Item3.ConsiderTotalPenalty,
                                     data.Item3.ConsiderAutoBusinessShares,
+                                    data.Item3.ConsiderCustomModifiers,
                                     ctx.GetMemberUnderDelivery(year, m.MgNr).GetAwaiter().GetResult()
                                 ) { Date = postalDate })];
                             } catch (Exception) {