Will there be any surprises after I sign up?

If you sign up on June 15, both your included GB and monthly fee for the first month will only be half of what the selected plan specifies. And if you signed up on June 29, you would only get 1/30th of the included data and only pay 1/30th of the base fee. When the new month starts, you would gain access to the full amount of data included in the plan, and pay the full base fee associated with the plan (plus any overage fees) one month later.