With the Starter membership, you have the flexibility to choose items at discounted prices every month. Every month, you will get charged for the membership, but you do not have to choose items every month! If you do not spend all of your credits for one month, it rolls over to the next month. Just make sure to leave your add-ons blank if so!
However, if you cancel your membership, please take a look here for more information.
For more information about the Starter subscriptions, please take a look here.