According to the Riverlink toll page, it's an extra dollar for the non-registered toll by plate. If you are worried about it setup a toll by plate account, then close it after 30 days or so. I did this in Washington to simplify our crossings and they just mailed me a check for the balance when I closed the account. I've even registered my vehicles in Vancouver BC for the toll by plate bridges, just for the savings - same thing, closed the account after 30 days.
Unless you are going to be on a lot of US EZPass type roads, it not worth getting their EZPass friendly transponder. I'd just either register for the toll by plate account or just let it ride and wait for the bill in the mail.