Here's how you beat the system : Use software to control you connections (don't do the PPPoE using the modem). Use something like
rp-pppoeand simply have a cron job that re-connects you at say 0210 (10 mins into the "unlimited period") and then again at 0750 (10 mins before the period ends) and schedule all your downloads in that session.
Done. Download all you want and you won't have to bother about excessive/wrong billing.