When sending within the same country, or between countries that are members of the European Union, your shipment will not require customs clearance.
If you are sending internationally between countries that are not both member states of the European Union your shipment will be subject to customs clearance and you will be required to complete a pro forma invoice detailing the contents and estimated used value of your goods for customs purposes.
You will be asked to fill this out once you have placed your booking.  To avoid delays, customs documentation should also be attached securely to your luggage or box so that it is clearly visible to the customs authority.