There are two types of invoices accepted for customs: Commercial and Proforma. A Commercial Invoice should be used when the goods were part of a commercial transaction. A Proforma Invoice would be used if the goods have no commercial value (e.g. not for resale) or personal shipments.
Both invoices are required to declare where the work is coming from and going to, what it is made of, and the value. This form is needed for both export and import customs declaration, as it helps keep track of what is owed for duties and taxes and the party responsible for paying. The invoice also helps to track what is coming in and out of the country.