BH, I've had to deal with several errant merchants over the past 2 years. Send a certified letter to the the card company contesting the charge. If at all possible, include any and all documentation (from their website, maybe?) that led you to believe it was quarterly or would be prorated. So far, I've nuked every inflated charge and one merchant didn't even bother to resubmit the corrected bill (I really nailed them in a bait-and-switch scheme.).