On Abakion App Management, sub-page Abakion Feature Management, you need to enable the Feature Code "Show Customs Tariffs".
Remember to set up Tariff No. on the Item, before generating your Sales Order & Warehouse Shipment.
Customs tariff calculation can be done on Sales Order, Warehouse Shipment and Posted Warehouse Shipment.
Calculate Value on Customs Line
See user videos about the functionality here: