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Tell them the weight was 12.8 oz, itemize everything in the shipment down to the 1/10th ounce (including dunnage), explain that the weight was confirmed by the scale at the post office, and now you'll need to notify the Regional Postal Inpector to come get the shipment and its contents to file a claim. Please listen for the doorbell.
If they are truthful, they will understand and cooperate (at which point, call off the dogs and just reship the item).
If they are scamming you, they'll whine about your terrible customer service and go away. It probably isn't the first time they've done this trick, but if you make it easy on them, they'll just keep doing it.
Each item is scanned during the packing process. So it is close to impossible to forget an item or ship a wrong item.