diff --git a/src/BusProNet/Constants.php b/src/BusProNet/Constants.php
index 022556c..f415cb9 100644
--- a/src/BusProNet/Constants.php
+++ b/src/BusProNet/Constants.php
@@ -47,4 +47,8 @@ final class Constants
     public const STATUS_AVAILABLE = 'Frei';
     public const STATUS_BLOCKED = 'Buchungsstop';
     public const STATUS_ON_REQUEST = 'Anfrage';
+
+    // Payment methods
+    public const PAYMENT_METHOD_TRANSFER = 'transfer';
+    public const PAYMENT_METHOD_DEBIT = 'debit';
 }
diff --git a/src/Form/BookingParticipantType.php b/src/Form/BookingParticipantType.php
index a90a90f..790e421 100644
--- a/src/Form/BookingParticipantType.php
+++ b/src/Form/BookingParticipantType.php
@@ -176,26 +176,7 @@ class BookingParticipantType extends AbstractType
      */
     private function removeExcludedFields(FormInterface $form, BookingDtoInterface $bookingDto, int $participantIndex, array $formData = []): void
     {
-        $dynamicFields = [
-            'assignedRoomId',
-            'remarksRoom',
-            'courses',
-            'additionalServices',
-            'board',
-            'rentals',
-            'rentalInsurance',
-            'skiPass',
-            'transportationOutbound',
-            'transportationInbound',
-            'pickupOutbound',
-            'pickupInbound',
-            'parking',
-            'licensePlate',
-            'bulkInsuranceBooking',
-            'insurance',
-        ];
-
-        foreach ($dynamicFields as $fieldName) {
+        foreach (ParticipantDto::DYNAMIC_FIELDS as $fieldName) {
             if ($form->has($fieldName) && !$this->fieldStateProvider->shouldIncludeField($fieldName, $bookingDto, $participantIndex, $formData)) {
                 $form->remove($fieldName);
             }
@@ -224,12 +205,7 @@ class BookingParticipantType extends AbstractType
         $this->addBaseFields($form, $bookingDto, $participantIndex);
 
         // Rebuild dynamic fields
-        $dynamicFields = [
-            'assignedRoomId', 'remarksRoom', 'courses', 'additionalServices', 'board', 'rentals', 'rentalInsurance',
-            'skiPass', 'transportationOutbound', 'transportationInbound', 'pickupOutbound', 'pickupInbound', 'parking',
-            'licensePlate', 'bulkInsuranceBooking', 'insurance',
-        ];
-        foreach ($dynamicFields as $fieldName) {
+        foreach (ParticipantDto::DYNAMIC_FIELDS as $fieldName) {
             if ($form->has($fieldName)) {
                 $form->remove($fieldName);
             }
diff --git a/src/Form/Service/ParticipantFieldHandlerRegistry.php b/src/Form/Service/ParticipantFieldHandlerRegistry.php
index 1935aec..a26b136 100644
--- a/src/Form/Service/ParticipantFieldHandlerRegistry.php
+++ b/src/Form/Service/ParticipantFieldHandlerRegistry.php
@@ -294,7 +294,9 @@ class ParticipantFieldHandlerRegistry
     {
         // Sync fields for all registered handlers
         foreach ($this->handlers as $fieldName => $handler) {
-            if (property_exists($participant, $fieldName)) {
+            // Only sync fields that were in the original submission
+            // This prevents adding extra fields that would cause validation errors
+            if (property_exists($participant, $fieldName) && array_key_exists($fieldName, $participantData)) {
                 $dtoValue = $participant->{$fieldName};
                 $participantData[$fieldName] = $this->convertDtoValueToSubmittedFormat($dtoValue);
             }