It's perfectly normal to cancel your subscription, you can do this here: https://app.anymailfinder.com/settings/billing
There is no contract, you can cancel at any time.
If you cancelled straight after you bought the credits you'd still get to keep using your credits for that billing month.
If do you cancel, you don't get to keep your rollover credits after that billing month. If you subscribe to a monthly plan, all unused credits rollover month after month.
There is an option to pause your monthly subscription for 1 month. Once you click cancel, you'll also see an option to freeze your account and payments for 1 month after which it will auto-restart.