This is a hypothetical question, but say I run an email hosting service and I want to charge my customer a fee per month for each email account that he has set up. Let's say that the email accounts are $1 per month each and when he first signs up he had 5 email accounts, but gradually adds more each month. Is there a recurring billing service that would allow me to add and subtract from the total recurring charge instead of just setting the fee during the initial account setup?
I'm also a bit confused about the need for a payment gateway and a merchant account. I'd really like the payment and all of the legal requirements that go with that to be offloaded to this service if possible.
Have you seen Chargify's component pricing? http://chargify.com/blog/new-feature-quantity-based-components/ And Chargify does not replace a merchant account and gateway. It's just a billing system. You'll still need to get a merchant account and gateway to accept credit cards.
I know that CheddarGetter is set up to add tracking information to all your billing plans; so, in your case, you would be able to set up an automatic piece of code that would see when someone increased or decreased their emails, and CheddarGetter would change their plan accordingly. We use CheddarGetter for a SaaS we're about to release, and we have it set up in a similar way. It was very easy to implement!
Also, we're using CheddarGetter for both the billing management software and the billing gateway. All we had to do on the side was get the Merchant Account, which made it a lot less of a hassle than if we had to get all three.