Take a look at the offerings from:
Typically these services provide ways for YOU to charge YOUR customers. But maybe they have options so that your customers can charge their customers.
I have issues with this type of service because many of them lock you into their choice of gateway providers, their APIs, and their price increases.
Recently Chargify completely did away with their free tier, causing all the small companies who had invested tons of time implementing the Chargify API to have to either start paying for the service, or scrap all the work they had done. Luckily I was still just evaluating them, so I wasn't burned. But they did go from top on my list to bottom overnight.
I just came across a new one, but don't know much about them (are they PCI compliant, are they stable, will they be around in a few years, etc.). On paper they look good, as they support numerous gateways and offer a migration tool from the others. See them at http://zingybill.com/