I only want 30 days of cellular data. Why does the recurring domestic plan options say my credit card will be billed every 30 days?

If you're in the U.S., AT&T offers a month-by-month plan. The point of the agreement is that if you don't take action to cancel your plan, the default is your card will be billed every 30 days. To end the plan, you must cancel it. When you're ready to cancel, there may be a way to do this online (at http://www.attwireless.com) - I don't know. But you can phone AT&T at (800) 331-0500 to speak to a service person there. That numbet is staffed 24 hours a day.