The only was you can stop that sort of loophole exploitation is by massive enforcement (i.e. large numbers of random package investigations to establish the actual value of the shipment and comparing it with the declared value).

The cost of such enforcement is very high - possibly not as high as the value of the goods imported by lying about their value, but there's actually no way to know that (since we don't know the actual value of goods that entered under the de minimis exception).

If significant parts (either in numbers or purchasing power) of your population are willing to lie to break the law, you've got problems larger and more widespread than the de minimis exception.