Trouble posting to plugin controller
Does the request requires login?
If it doesn't, you need to add the exception on the controller
protected $allowAnonymous = array('actionSubscribe');

Also, did you try adding admin to your URL?
&lt;input type="hidden" name="action" value="/admin/actions/paypalSubscription/subscription/subscribe"&gt;

I was trying to post to an action once through the front-end and it only worked after adding /admin