If you are being picked up at the end of your trek, you can return the key to the driver on pick up.
If you are walking West to East you can leave your key in your Standley Chasm container as this is the last food drop.
If you have finished your trek in Alice Springs you can leave your key at the hotel reception in an envelope with LTTS and your name written on it and SMS us that it's ready for collection. If you have gear retrieval you can leave your key in the container after you have finished with it.