You can pay for the connection at will. Once you've started it for a given month, you've paid for the month (and I believe that is a month from when you turn it on), then turn it off and at the end of the month you won't be billed until you turn it on again (turn it on is identified as a deliberate "I want to pay the bill and connect" type action, not an accidental connect. It's all quite civilized and clean. I have not actually turned it off because I'm one of those with unlimited service that I'm loath to lose, but what I've stated is my understanding and expectation.
To clarify, "at will" is in one month increments.
Also, as I understand it you don't have to wait till the end of the month to cancel. You can cancel early your data will still be active till the end of the month, or till you use up your allotment, whichever comes first.
Michael