When Customs challenges your declarations, how you respond decides whether you're liable
Customs actively checks that importers are paying the correct duty. They detain shipments to assess declarations, and where they believe goods have been declared incorrectly, they instruct the importer to amend the declaration, bring the extra duty to account, and pay a penalty on top.
They don’t stop at the current shipment either. Customs routinely inspect your past imports going back two years, which is where a single disagreement can turn into a demand for historic duty across multiple entries. How you answer that challenge, and how well it’s motivated, is what decides whether the matter goes away or turns into a substantial debt.