Starting July 1, 2021, the European Union requires tariff numbers, called HS (Harmonized System) Codes, for products shipped to EU countries. These codes help to maintain the administrative procedure for cross-border shipment inventory. So all orders going through customs require HS codes; otherwise, the shipment can get stuck.
If your ecommerce platform, such as Etsy, asks for these codes, you can view or download this spreadsheet of all Printful products and their HS Codes (last updated on January 11, 2022) and search for the products you sell.