Repeat invoicing does work. The only issue is from what I can tell, is there is no link to put in the email so the customer can easily get to the billing page. Also, if a client logs into DNP, it isn't obvious where to go to pay a bill.
The worst aspect of the ecommerce solution is in the situation a customer needs more resources, such a disk space or bandwidth or more emails. In this case you cannot simply buy a configured add-on. You cannot buy this from the store. Don't ask me why. This one caught me last year.