Braintree and Stripe both allow you to store card numbers without creating a subscription. I would either go the prepayment route (allow the customer to re-loa

Braintree and Stripe both allow you to store card numbers without creating a subscription. I would either go the prepayment route (allow the customer to re-load a balance, then draw it down with usage), or set billing/time thresholds (bill the balance every X days or whenever the total hits $Y). I use several API services that use the reloading method, and Google Adwords does the billing/time threshold.