We only charge you in months where you're paid through Squirrel - i.e. your salary arrives into your Squirrel account. If you aren't paid through Squirrel in a given month, then we won't charge you - you only pay when you use the service.
We'll automatically deduct the amount from your salary when it comes in, so there's no hassle managing cards etc on your account.
You can see how much your subscription is in the "profile" section of your account.