Brian,
In DotNetInvoice a recurring invoice is setup once for a specific amount and is then billed every X days/months/years for the same amount. If the client has a credit card on file the card is automatically charged, otherwise they are sent an invoice via email they can login and pay. But the amount entered when the invoice is first created is billed each month - it can't vary from month to month.
DNI does include a credit card form for the client to pay their invoice the first time it's issued. Remember, this is invoicing software rather than a shopping cart, so you as an administrator have to create an invoice that is then emailed to the client. They can then login to their client account to pay the invoice. Each month after that it will be automatically charged.
Does this answer your questions?
---
Rob
DotNetInvoice Support