Invoices can be in four different statuses, and also have attributes that affect if its editablie or not.
The invoice has not get received a number. It needs to be exported to Fortnox to receive one.
The invoice is exported to Fortnox and can now be viewed as an PDF document.
The invoice has been cancelled and can no longer be edited.
The invoice has been credited and can no longer be edited.
Aside from its status, invoices have these related attributes.
Sent invoices can only edited by administrators.
Booked invoices can not be edited and needs to be credited if changes are necessary.
When an invoice payment in booked in Fortnox, the invoice in marked as paid.