A one time buy allows client to buy from 100 to 20000 credits. Bought credits are valid for 90 days (they can be used in the following 90 days). Each consecutive one time buy extends all the existing credit validity by 90 days (credit validity periods do not stack). So if a client makes two one time purchases in the same day the validity of all credits will be extended by additional 90 days (not 180). A one time buy is useful if you have a subscription and you need some extra credits in the current month but you want to keep your active subscription.