Data Transfer Query

[linnea]

I just installed private tunnel. I noticed that in your payment page there is list of data transfer with price. $12 for 50gb.
Then on the top it says: Pay only for data you transfer. No monthly fee. I have two question.

1. How will I know the number of data transfer I use?
2. What will happen if I didn't consume my 50gb for a year; Do I need to pay another $12 for another year?

Thanks in advance!