Monthly plans vs One-time plans
Do I have to pay monthly or one time?
When can I cancel a monthly plan?
Whenever you want. The credits you still have available for the current billing period will be available until the end of the billing period. At the end of it, you won't be billed again and the plan will be reset to the free plan.
In what order are the credits used?
The monthly credits are always used first and when those credits are exhausted then the one-time credits, if available, will be used.
This means that if you have both a monthly and a one-time plan the one-time credits will only be used when needed.