Monthly POP Credits do not roll over.
Monthly POP Credits are credits you receive from your PageOptimizer Pro subscription. Your subscription is for monthly access to POP. Once your subscription renews, the subscription will refill up to the amount the plan is for.
For example, the PageOptimizer Pro Basic Plan; is for a total of 20 Monthly POP Credits. Upon resubscription, it will refill you up to 20 credits. Even if you have credits left over from the month prior.
If you cancel your POP subscription, the current amount of credits that you have in your account will not be lost. Until your resubscription date, when they will disappear.