Be sure to consider the accounting & customer service costs of whatever model you choose. A one-time fee is is easy to administer, while a recurring cost is an order of magnitude more complex, and recurring costs with roll-over and top-up ability yet another order of magnitude more complex-- And not just from your side, but the mental burden it inflicts on the user. Recurring costs, no matter how small, always present a greater mental barrier to purchase than a one-time fee.