Can I also send a letterbox parcel in a letterbox?

You can send a letterbox parcel by presenting it at the carrier's counter, where you will receive a shipping receipt. From here, it becomes possible to track and trace this shipment.
 

With PostNL, it is also possible to send a letterbox parcel via the mailbox. However, a letterbox parcel sent via the mailbox does not receive an 'acceptance scan', and the Track & Trace expires if it is not registered.