If you have a "continuous authority" facility enabled on your merchant account you can make arbitrary charges on card numbers authorised and stored by your payment processor. CA doesn't seem too hard to acquire, at least here in the UK it was easier than we expected at worldonahanger.com. The downside with this approach is that in the event that you want to switch payment processors, you have to ask your customers to resubmit their card details for reauthorisation (as far as I can tell).