You are at liberty to purchase the amount of SMS credits you require. Credits are available in bundles varying in size and amount.Full pricing details are available here: https://inforumobile.co.uk/pricing/.
Once bought the credits are at your disposal for an unlimited amount of time. You simply use them at your own pace. When exhausted you are free to top up your account by buying an additional stock as per your needs.
There are no subscription fees, fixed or recurring charges involved.