A shipment coming from a country outside the European Union can be subject to a check and various duties and fees: VAT, import duties (also called “customs duties”) and excise duties (only on specific products such as alcohol, tobacco, etc.).
“Customs formalities” are administrative formalities concerning the collection of the various duties and fees.
Some Internet traders give an estimation of the customs duties and formalities beforehand as soon as the order is placed.
Some postal operators will ask a compensation for performing those customs formalities, payable in addition to the duties and fees; others have already integrated those costs in the fixed costs of their products/services.