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.