Hook to payment in my plugin

#1

I am trying, unsuccessfully so far, to hook to s2Member payments in my own plugin. In other words, any time there is a payment through s2Member and Paypal, to be able to act on it in my own plugin.
In my plugin I have:
`

<?php add_action('ws_plugin__s2member_during_paypal_notify_during_subscr_signup_w_update_vars', 'st_signup'); function st_signup($vars = array()) { /* Just logging at this point */ $file = dirname(__FILE__) . '/hook.log'; file_put_contents($file, file_get_contents($file) . print_r($vars, true)); }` but the hook never fires. I have verified in 'plugins_activated' that my plugin loads after s2Member. Any help will be much appreciated.
#2

Don’t use hook, use “Notification URL” instead. Sure, this URL must be saved in notifications, but it fires always, and with any gateway.

#3

Ok, I can try that. It just seem like a round about way of doing things.