Absolutely! When you book a class and you don't have enough credits, there are two options.
You will see a pop-up stating that you do not have enough credits when you select a class. It will give you the option to go ahead and book in the class and be charged for the additional credits that are required.
You can also buy additional credit packs. This can be done on the Billing page.
If you would ever like to upgrade your plan to allow for more monthly visits, you can do so on the Billings page under "Your Plan."