If you pick up more than 30 minutes early then the computer system assumes you are still dropping off at the originally scheduled return time so you'll see the contract amount increase. This is nothing to worry about. Just return on your scheduled return day at or before (actually there's usually a 29-minute grace period) the time you originally picked up and you'll be charged your originally contracted amount. This often confuses people but it's just the counter system making certain assumptions and is nothing to worry about.