The reality of international shipping is that sometimes you will be charged a tariff. inFlow can include that amount in the total cost of your item (placing it in the product record for profit calculations later) if you enter it into the non-vendor costs field on your purchase order. The non-vendor costs field can be found in the bottom left hand corner of your purchase order screen.

In advanced mode it will be found on the Order tab, in simple mode it is found on the Purchasing tab (as below):