How do you transition from month to month?
Here's a scenario.

Let's say they paid for a plan on the 1st of the month and they rented a movie on the 29th of that month. Instead of returning it on the 31st, they returned it "late" on the 2nd of the following month.
If they renew the following month, I guess it wouldn't be a problem. But what if they don't?
Also, do you give them the option to renew their plan automatically if they used a credit card to pay the first time? What if they pay in cash for only that one month?
Have you written up a subscription plan policy? Perhaps you can share that with everyone.

- Tek