diff --git a/CRM/Extrafee/Fee.php b/CRM/Extrafee/Fee.php
index 570a8bc..c3b628a 100644
--- a/CRM/Extrafee/Fee.php
+++ b/CRM/Extrafee/Fee.php
@@ -41,7 +41,7 @@ public static function displayFeeMessage($form, $extraFeeSettings) {
   }
 
   public static function addOptionalFeeCheckbox($form, $extraFeeSettings) {
-    $form->add('checkbox', 'extra_fee_add', $extraFeeSettings['label']);
+    return $form->add('checkbox', 'extra_fee_add', $extraFeeSettings['label']);
   }
 
   /**
@@ -73,14 +73,15 @@ public static function modifyTotalAmountInParams($formName, &$form, $extraFeeSet
         $form->set('amount', $form->_amount);
       }
     }
-    elseif ($formName == 'CRM_Event_Form_Registration_Register') {
+    elseif ($formName == 'CRM_Event_Form_Registration_Register' || $formName == 'CRM_Event_Form_Registration_AdditionalParticipant') {
       $params = $form->getVar('_params');
-      if (!empty($params[0]['amount'])) {
-        $params[0]['amount'] += $params[0]['amount'] * $percent/100 + $processingFee;
-        $params[0]['amount'] = number_format($params[0]['amount'], 2);
-        $form->setVar('_params', $params);
-        $form->set('params', $params);
+      foreach ($params as $key => &$value) {
+        if ($key == array_key_last($params) && !empty($value['amount'])) {
+          $value['amount'] += $value['amount'] * $percent/100 + $processingFee;
+        }
       }
+      $form->setVar('_params', $params);
+      $form->set('params', $params);
     }
   }
 
@@ -118,7 +119,7 @@ public static function isFormEligibleForExtraFee($form, $extraFeeSettings, $ppEx
    * Get Extra fees overriden by payment processor.
    */
   public static function getProcessorExtraFees() {
-    $ppExtraFeeSettings = json_decode(Civi::settings()->get('processor_extra_fee_settings'), TRUE);
+    $ppExtraFeeSettings = (array) json_decode(Civi::settings()->get('processor_extra_fee_settings'), TRUE);
     foreach ($ppExtraFeeSettings as $ppID => $pp) {
       if (empty($pp['percent']) && empty($pp['processing_fee'])) {
         unset($ppExtraFeeSettings[$ppID]);
diff --git a/extrafee.php b/extrafee.php
index b016f21..6011fc3 100644
--- a/extrafee.php
+++ b/extrafee.php
@@ -57,7 +57,11 @@ function extrafee_civicrm_buildForm($formName, &$form) {
       'template' => "{$templatePath}/CRM/Extrafee/Form/processor_extra_fee.tpl"
     ));
   }
-  if (!in_array($formName, ['CRM_Contribute_Form_Contribution_Main', 'CRM_Event_Form_Registration_Register'])) {
+  if (!in_array($formName, [
+    'CRM_Contribute_Form_Contribution_Main',
+    'CRM_Event_Form_Registration_Register',
+    'CRM_Event_Form_Registration_AdditionalParticipant'
+    ])) {
     return;
   }
   $extraFeeSettings = json_decode(Civi::settings()->get('extra_fee_settings'), TRUE);
@@ -65,9 +69,26 @@ function extrafee_civicrm_buildForm($formName, &$form) {
   if (!CRM_Extrafee_Fee::isFormEligibleForExtraFee($form, $extraFeeSettings, $ppExtraFeeSettings)) {
     return;
   }
+  $addOptionalCheckbox = TRUE;
+  $freeze = FALSE;
   if (!empty($extraFeeSettings['percent']) || !empty($extraFeeSettings['processing_fee'])) {
+    $params = $form->getVar('_params');
+    if ($formName == 'CRM_Event_Form_Registration_AdditionalParticipant' && !empty($params[0]['payment_processor_id'])) {
+      $defaults['extra_fee_add'] = 1;
+      $form->setDefaults($defaults);
+      $freeze = TRUE;
+      $form->assign('selected_payment_processor', $params[0]['payment_processor_id']);
+      if (!empty($params[0]['extra_fee_add'])) {
+        $addOptionalCheckbox = $freeze = FALSE;
+      }
+    }
     CRM_Extrafee_Fee::displayFeeMessage($form, $extraFeeSettings);
-    CRM_Extrafee_Fee::addOptionalFeeCheckbox($form, $extraFeeSettings);
+    if ($addOptionalCheckbox) {
+      $element = CRM_Extrafee_Fee::addOptionalFeeCheckbox($form, $extraFeeSettings);
+      if ($freeze) {
+        $element->freeze();
+      }
+    }
   }
 }
 
@@ -87,7 +108,11 @@ function extrafee_civicrm_postProcess($formName, &$form) {
     Civi::settings()->set('processor_extra_fee_settings', json_encode($ppExtraFeeSettings));
   }
 
-  if (!in_array($formName, ['CRM_Contribute_Form_Contribution_Main', 'CRM_Event_Form_Registration_Register'])) {
+  if (!in_array($formName, [
+    'CRM_Contribute_Form_Contribution_Main',
+    'CRM_Event_Form_Registration_Register',
+    'CRM_Event_Form_Registration_AdditionalParticipant'
+    ])) {
     return;
   }
   $extraFeeSettings = json_decode(Civi::settings()->get('extra_fee_settings'), TRUE);
@@ -96,6 +121,10 @@ function extrafee_civicrm_postProcess($formName, &$form) {
     return;
   }
   $ppID = $form->getVar('_paymentProcessorID');
+  $params = $form->getVar('_params');
+  if ($formName == 'CRM_Event_Form_Registration_AdditionalParticipant' && empty($ppID) && !empty($params[0]['payment_processor_id'])) {
+    $ppID = $params[0]['payment_processor_id'];
+  }
   if ((!empty($extraFeeSettings['percent']) || !empty($extraFeeSettings['processing_fee'])) && !empty($ppID) && empty($form->_ccid)) {
     CRM_Extrafee_Fee::modifyTotalAmountInParams($formName, $form, $extraFeeSettings, $ppID);
   }
diff --git a/templates/extra_fee.tpl b/templates/extra_fee.tpl
index 040acfc..90d8aaf 100644
--- a/templates/extra_fee.tpl
+++ b/templates/extra_fee.tpl
@@ -3,6 +3,7 @@
 CRM.$(function($) {
   var processor_extrafee = {/literal} {if $processor_extra_fee_values} {$processor_extra_fee_values} {else} 0 {/if}{literal};
   var extra_fee_settings = {/literal} {if $extra_fee_settings} {$extra_fee_settings} {else} 0 {/if}{literal};
+  var selected_payment_processor = {/literal} {if $selected_payment_processor} {$selected_payment_processor} {else} '' {/if}{literal};
 
   var isQuickConfig = {/literal}{$quick_config_display}{literal};
   var payNowPayment = {/literal} {if $payNowPayment} {$payNowPayment} {else} 0 {/if}{literal};
@@ -35,7 +36,9 @@ CRM.$(function($) {
     $('#pricesetTotal').append(optional_input + msg);
   }
 
-  $('input#extra_fee_add').on('change', function() { displayTotalAmount(calculateTotalFee()); });
+  $('input#extra_fee_add').on('change', function() {
+    displayTotalAmount(calculateTotalFee());
+  });
 
   /*
    * Thanks: https://stackoverflow.com/a/59268677/11400326
@@ -60,7 +63,7 @@ CRM.$(function($) {
     ( totalfee < 0 ) ? $('#pricelabel, #pricevalue').hide() : $('#pricelabel, #pricevalue').show();
   }
 
-  function formatExtraFee(amount, c, d, t){
+  function formatExtraFee(amount, c, d, t) {
     var n = amount,
       c = isNaN(c = Math.abs(c)) ? 2 : c,
       d = d == undefined ? "," : d,
@@ -87,6 +90,9 @@ CRM.$(function($) {
     var pp = $('input[name=payment_processor_id]:checked').val();
     if (typeof pp === 'undefined') {
       pp = $('input[name=payment_processor_id]').val();
+      if (selected_payment_processor && typeof pp === 'undefined') {
+        pp = selected_payment_processor;
+      }
     }
     if (typeof pp !== 'undefined' && pp != 0 && totalFee) {
       if (addExtraFee) {