Hi, is there a way to create an automatic interest on overdue invoices?Or must I do each manually?

Good day, debiteure.

You'll have to manually add the interest on overdue invoices. You can create or set up the interest as a service item in QuickBooks Online (QBO).

1. Go to the Gear icon and choose Products and Services.
2. Click on New and select Service.
3. Enter the name of the item and choose an income account.
4. You can also enter the rate of the item.
5. Click on Save and close once you're done.

You'll also want to create templates for recurring transactions in QBO.

We'll be around if you have further questions. Have a good one.

@debiteure 

There are some 3rd party AR management app in the market with automatic interest feature.  Consider to integrate it with your QBO.