If they were deceiving you just to get the second pair free, it seems odd that they'd give you a neg and claim the second rat traps weren't received. They would have probably been laughing all the way to glowing positive feedback for you. Perhaps there was a problem sending rat traps through the mail and it was intercepted somewhere.

As everyone says, tracking is good but proof of receival via a signature using Registered Post is the way to go. You can then follow up with Aus Post and see if they really did receive it by checking if they signed for it. Since your traps are over $20, as a seller you need the safety of knowing whether it was received or not.