It seems as though the callback to ZC after loggin in to PayPal pressing continue in PayPal throws the following error on 'postback':

403 Permission Denied
You do not have permission for this request /store/ipn_main_handler.php?type=ec&token=EC-1PB63163DD995290N&PayerID=D5BLAHHHHB84

in my mind it seems like a simple permissions issue, but even setting 755 on that file doesn't solve...any ideas?