Single month memberships are sold in the Roblox Mobile App available on the iTunes, Google Play, and Amazon app stores.
Memberships bought through the Roblox website are renewing by default. You are able to cancel at any time before the renewal date. Even if you cancel the membership you will get the benefits until the expiration date.
Memberships are non-refundable. To avoid a recurring charge it's better to cancel sooner rather than later. Read more about how to cancel.