No hidden charges with us! The 20 GB for $20 Data Plus option is going to cost you $20 /mo (including GST) as long as it remains active. Not more, not less!
You’ll be billed in advance every month, except for the first month where you will be billed either immediately if you are a existing subscriber or on your first bill if you are a new subscriber. More detail on the 20 GB for $20 Plus option billing policy here.