try something else

booking.stripe.js

@@ -24,7 +24,7 @@ jQuery(document).ready(function($) {
     });
 
     //attempt to update amount for amex cards
-    $('#card_number').blur(function() {
+    $('#card_number').focusout(function() {
         console.log(['Card field lost focus', $(this).attr('class')]);
     });