actually use the class I define in trans

apps/addons/templates/addons/paypal_thanks.html

@@ -10,11 +10,11 @@
     <h2>{{ _('Purchase Add-on') }}</h2>
   {% endif %}
   <h5>{{ _('Thank you!') }}</h5>
-  {% trans addon_name=addon.name, manifest_url=addon.manifest_url,
+  {% trans addon_name=addon.name, manifest_url=addon.manifest_url if addon.is_webapp() else '',
           class='trigger_app_install' if addon.is_webapp() else 'trigger_download',
           href='#' if addon.is_webapp() else download %}
     Your purchase of <strong class='addon-title'>{{ addon_name }}</strong> is complete.
-    <p>If installation does not automatically begin, please <a class="trigger_app_install" data-manifest-url="{{ manifest_url }}" href="{{ download }}">click here</a>.</p>
+    <p>If installation does not automatically begin, please <a class="{{ class }}" data-manifest-url="{{ manifest_url }}" href="{{ download }}">click here</a>.</p>
   {% endtrans %}
   <p class="mozilla-mission">
   {% trans learn_more='http://www.mozilla.org/causes' %}