There are several possibilities for customs withholding / high fines:
1) If the goods need BIS, WPC and other certificates, but the shipper and recipient cannot provide them
2) The goods have been cleared by the customs and the recipient refuses to pay customs duties
3) The consignor or recipient fails to upload KYC as required by the customs
4) The customs considers that the report is concealed or understated
Other items that the customs deems necessary to be detained for inspection