What redirection link do you get after payment? Could you take a screenshot of it and post it here?
Make sure you have logging enabled, run another test purchase and show me the log entries x'ing out any private info, please.
WP Admin -> s2Member -> PayPal Options -> Account Details -> Enable Logging Routines
/wp-content/plugins/s2member-logs/Statistics: Posted by Cristián Lávaque — August 10th, 2011, 1:45 pm
]]>