As of July 1, 2012, the law standardizing extrajudicial collection costs came into effect. As of this date, in the final demand letter to your customer, you are not only required to use a 14-day payment term, but you are also required to state the maximum amount that will be charged in collection costs if payment is not made within the 14 days. If you cannot offset VAT, the collection costs will be increased by the amount of VAT.
You can also upload a CSV file to perform a bulk calculation. Here you are given the choice of which column to apply the calculation to.