Ask HN: Is there a way to bill automatically for a metered service?

If you sell a metered service, like Heroku or AWS do, are there automated ways of billing your customers' credit cards?

Normally, using any one of a number of recurring-billing options would do the trick. But it seems like these are largely intended for cases where the amount that gets spent doesn't change month-to-month.

What if the amount does change month-to-month? Are there processors or payment services that can accommodate that sort of business model?