The x_recurring_billing doesn't really do a lot, it is simply a flag that tells Authorize.net that the payment *should* be recurring. it does not pass a time frame or amount, so you would need to manually convert the transaction by logging into your Authorize.net account.
There is a separate API for Recurring billing that Authorize.net has developed, eCart does not support this API however. You would need to hand code the logic to process the transaction using the recurring billing API.