The local Apple AU trade-in web page here says this:
How do I pack my device for trade-in?
You’ll receive detailed instructions on how to prepare your device so you can complete your trade-in.
If you’re trading in your device for credit towards a new one, you can choose to drop it off or schedule a courier pick-up. If you prefer to drop it off, we’ll email you a barcode you can take to your local Australia Post, where they’ll pack up your device and send it to our trade-in partner. If you select courier pick-up, we’ll send you a trade-in kit so you can pack up your own device, and a courier will come to collect at the scheduled time.
To trade in a device without purchasing a new one, you’ll use the courier pick-up method described above.
So you must have received some detail in the "trade-in kit". Were you provided some sort of barcode to stick on it for the courier?
Additionally, there is a phone number in the last FAQ item:
How do I get additional help with my trade-in?
Contact us on 133-622.