How do I merge a credit note and the invoice on one 'booking', without the need for a transaction?

Follow the steps below:

1. Upload both documents in OkiOki.
2. Choose the invoice and follow … take me to the booking.
3. In "Documents" choose the “+” icon at the top right and select the credit note.
4. Add a comment, e.g. “Invoice and Credit note, without transaction”.
5. Then choose “Complete!” and you're all set.

You can find more information on combining multiple documents and multiple transactions in this article.