Business logic/advantages aside for this particular analysis, what is the recommended user experience related to recurring, pre-paid billing when the customer's allocation has been exceeded: charging overage, or renew the subscription early for the next period?
(In this case, post-pay and service disruption--or disabling subscriptions until the next billing period like is typical with cell service--are not options.)
Lingo defined for clarity:
By recurring, I mean that this is a payment which needs to be made on a regular basis to keep the subscription active.
By pre-paid, I mean that this payment is made for the billing period ahead of time (before it is used).
By overage, I mean that the next cycle's payment is increased to compensate for usage over the allotted amount. Advantage: Payments are made on a regular basis. Disadvantage: Payments are variable amount depending on usage.
By renewing early, I mean that the subscription is renewed for another billing period as soon as its allocation is depleted or exceeded. In other words, you pay regularly at each billing period, or whenever you use up all your allotment, whichever comes first. Advantage: Payments are the same amount. Disadvantage: Payments may occur on an irregular basis depending on usage.