[mothered]

I was wondering if it would be better if I did it as a "spare device not in the box" or "my faulty device was stolen during shipment". I heard that if a replacement device is stolen during the transport the support would like to see an invoice from my "defective" device.

The empty box method (faulty device stolen during transit) will suffice.

In the event they request the receipt, tell them you've shipped It together with the faulty device (hence both device & Invoice stolen).