diff --git a/woo-business-central/admin/class-wbc-admin.php b/woo-business-central/admin/class-wbc-admin.php
index 587cf83..4902bde 100644
--- a/woo-business-central/admin/class-wbc-admin.php
+++ b/woo-business-central/admin/class-wbc-admin.php
@@ -94,6 +94,9 @@ class WBC_Admin {
         register_setting( 'wbc_orders', 'wbc_shipping_item_number', array(
             'sanitize_callback' => 'sanitize_text_field',
         ) );
+        register_setting( 'wbc_orders', 'wbc_default_customer_number', array(
+            'sanitize_callback' => 'sanitize_text_field',
+        ) );
     }
 
     /**
diff --git a/woo-business-central/admin/partials/wbc-admin-display.php b/woo-business-central/admin/partials/wbc-admin-display.php
index fd0d7aa..9c26a18 100644
--- a/woo-business-central/admin/partials/wbc-admin-display.php
+++ b/woo-business-central/admin/partials/wbc-admin-display.php
@@ -328,6 +328,19 @@ $tabs = array(
                                 </p>
                             </td>
                         </tr>
+                        <tr>
+                            <th scope="row">
+                                <label for="wbc_default_customer_number"><?php esc_html_e( 'Default Customer Number', 'woo-business-central' ); ?></label>
+                            </th>
+                            <td>
+                                <input type="text" id="wbc_default_customer_number" name="wbc_default_customer_number"
+                                       value="<?php echo esc_attr( get_option( 'wbc_default_customer_number', '' ) ); ?>"
+                                       class="regular-text" />
+                                <p class="description">
+                                    <?php esc_html_e( 'BC customer number to use for orders without NIF/CIF. Orders with NIF will look up or create a specific customer.', 'woo-business-central' ); ?>
+                                </p>
+                            </td>
+                        </tr>
                     </table>
 
                     <p class="submit">
diff --git a/woo-business-central/includes/class-wbc-customer-sync.php b/woo-business-central/includes/class-wbc-customer-sync.php
index b8eb660..dd3d8e4 100644
--- a/woo-business-central/includes/class-wbc-customer-sync.php
+++ b/woo-business-central/includes/class-wbc-customer-sync.php
@@ -20,24 +20,41 @@ class WBC_Customer_Sync {
     /**
      * Get or create a customer in Business Central
      *
+     * Lookup priority:
+     * 1. If NIF provided → search BC by taxRegistrationNumber, create if not found
+     * 2. If no NIF → use configured default customer number
+     *
      * @param WC_Order $order WooCommerce order.
      * @return string|WP_Error BC customer number or error.
      */
     public function get_or_create_customer( $order ) {
-        $email = $order->get_billing_email();
+        $nif = $order->get_meta( '_nif' );
+        $user_id = $order->get_user_id();
 
-        if ( empty( $email ) ) {
-            return new WP_Error( 'wbc_no_email', __( 'Order has no billing email.', 'woo-business-central' ) );
+        // No NIF → use default customer number
+        if ( empty( $nif ) ) {
+            $default_customer = get_option( 'wbc_default_customer_number', '' );
+
+            if ( empty( $default_customer ) ) {
+                return new WP_Error( 'wbc_no_nif_no_default', __( 'Order has no NIF and no default customer number is configured.', 'woo-business-central' ) );
+            }
+
+            WBC_Logger::info( 'CustomerSync', 'No NIF provided, using default customer', array(
+                'order_id'        => $order->get_id(),
+                'customer_number' => $default_customer,
+            ) );
+
+            return $default_customer;
         }
 
-        WBC_Logger::debug( 'CustomerSync', 'Getting or creating customer', array( 'email' => $email ) );
+        WBC_Logger::debug( 'CustomerSync', 'Getting or creating customer by NIF', array(
+            'nif' => $nif,
+        ) );
 
-        // Check if we have a cached BC customer ID for this user
-        $user_id = $order->get_user_id();
+        // Check cached BC customer number for this user
         if ( $user_id ) {
             $cached_customer_number = get_user_meta( $user_id, '_wbc_bc_customer_number', true );
             if ( ! empty( $cached_customer_number ) ) {
-                // Verify the customer still exists in BC
                 $exists = $this->verify_customer_exists( $cached_customer_number );
                 if ( $exists ) {
                     WBC_Logger::debug( 'CustomerSync', 'Using cached BC customer', array(
@@ -48,8 +65,8 @@ class WBC_Customer_Sync {
             }
         }
 
-        // Try to find existing customer in BC by email
-        $existing = $this->find_customer_by_email( $email );
+        // Search BC for customer with this NIF
+        $existing = $this->find_customer_by_nif( $nif );
 
         if ( is_wp_error( $existing ) ) {
             return $existing;
@@ -58,21 +75,20 @@ class WBC_Customer_Sync {
         if ( $existing ) {
             $customer_number = $existing['number'];
 
-            // Cache the customer number for this user
             if ( $user_id ) {
                 update_user_meta( $user_id, '_wbc_bc_customer_id', $existing['id'] );
                 update_user_meta( $user_id, '_wbc_bc_customer_number', $customer_number );
             }
 
-            WBC_Logger::info( 'CustomerSync', 'Found existing BC customer', array(
-                'email'           => $email,
+            WBC_Logger::info( 'CustomerSync', 'Found existing BC customer by NIF', array(
+                'nif'             => $nif,
                 'customer_number' => $customer_number,
             ) );
 
             return $customer_number;
         }
 
-        // Create new customer in BC
+        // Create new customer in BC with NIF
         $new_customer = $this->create_customer( $order );
 
         if ( is_wp_error( $new_customer ) ) {
@@ -81,14 +97,13 @@ class WBC_Customer_Sync {
 
         $customer_number = $new_customer['number'];
 
-        // Cache the customer number for this user
         if ( $user_id ) {
             update_user_meta( $user_id, '_wbc_bc_customer_id', $new_customer['id'] );
             update_user_meta( $user_id, '_wbc_bc_customer_number', $customer_number );
         }
 
-        WBC_Logger::info( 'CustomerSync', 'Created new BC customer', array(
-            'email'           => $email,
+        WBC_Logger::info( 'CustomerSync', 'Created new BC customer with NIF', array(
+            'nif'             => $nif,
             'customer_number' => $customer_number,
         ) );
 
@@ -96,13 +111,13 @@ class WBC_Customer_Sync {
     }
 
     /**
-     * Find customer in BC by email
+     * Find customer in BC by NIF (taxRegistrationNumber)
      *
-     * @param string $email Customer email.
+     * @param string $nif Tax registration number (NIF/CIF/NIE).
      * @return array|false|WP_Error Customer data, false if not found, or error.
      */
-    private function find_customer_by_email( $email ) {
-        $filter = "email eq '" . WBC_API_Client::escape_odata_string( $email ) . "'";
+    private function find_customer_by_nif( $nif ) {
+        $filter = "taxRegistrationNumber eq '" . WBC_API_Client::escape_odata_string( $nif ) . "'";
         $result = WBC_API_Client::get_customers( $filter );
 
         if ( is_wp_error( $result ) ) {
@@ -203,6 +218,12 @@ class WBC_Customer_Sync {
             'email'        => $order->get_billing_email(),
         );
 
+        // Add NIF as tax registration number
+        $nif = $order->get_meta( '_nif' );
+        if ( ! empty( $nif ) ) {
+            $customer_data['taxRegistrationNumber'] = $nif;
+        }
+
         // Add optional fields if present
         $phone = $order->get_billing_phone();
         if ( ! empty( $phone ) ) {