[mothered]

Simply tell them you've sent the Item back (obviously you'll do nothing of the sort).

Given It's still the busy time of year postal/carrier-wise, as far as they're concerned, It was lost In transit.

They'll probably request a tracking number.

There's always a way to circumvent anything and anyone.

@Skryptec has made some good points above, or you can simply say you've lost/misplaced the tracking number. After all, we're human and prone to error.