If your invoice schedule charges a fixed amount it is still possible to add extra costs (eg. Codabox). This can be done in two ways:
* In your price list create an item for the extra cost and tick the checkbox "In fixed amount"
In order to add costs to the invoice, of course, registrations must be made with that item.
* Another way is via the invoice scheduling:
Let's say you have a monthly fixed amount scheduling template set for 500€. Click the info-icon in front to open the schedule and change the invoice creation date.
At night an invoice proposal will be created. On the proposal you can add an invoice line.
When the invoice proposal is adapted as you wish you can manually number it.
More information about invoice scheduling can be found here.