PayPal seems to be having some issues with their IPN system. If you pay an invoice here, but it remains marked "Unpaid", please open a ticket with your PayPal transaction ID.
Update: The problem seems to be that PayPal is using SSLv3 to send IPN notifications to our billing system (WHMCS). Of course, with the recent POODLE vulnerability going public, we have disabled SSLv3. WHMCS and our staff are working on a workaround until PayPal comes up with something on their end.