The reality of international shipping is that sometimes you will be charged a tariff. You can enter this into the Non-vendor Costs field on your purchase order. This charge will not add on to the total of the purchase order itself, but will include that amount in the total cost of your item (placing it in the product record for profit calculations later) The non-vendor costs field can be found in the bottom left hand corner of your purchase order screen.

You can also find the sales equivalent for this in the bottom left of the Sales Order called Non-Customer Costs.