Okay…
It’s strange, because if the link generator works and you get a shortened URL, it should work the same after a purchase, the shortener is working.
If you change to TinyURL as you had it before, the problem goes away?
Can I see the log entries relevant to a test purchase that has that behavior? WP Admin > s2Member > Log Files
Is only the signup confirmation one failing, or are other emails not being sent by WordPress?
Thanks.