A: No. Interest invoices from Fortnox are not synced to Upsales. This applies to both the Fortnox and the Fortnox Premium app.
What is synced for overdue invoices: the fields Betald and Antal påminnelser on the order in Upsales are updated from the invoice in Fortnox, so you can see whether it is paid and how many payment reminders have been sent.