Within the EU and EEA, customs and import duties are a non-issue. Most other countries, too are pretty simple to ship to so we rarely, if ever see delays there. For outside EU shipments, trade documents detailing the contents of your shipment and its value.
If your country has import restrictions on certain goods, we'd recommend that you first contact support to check whether we'll be able to get items through to you. If we are unable to get an item through customs it'll end up returning to us.
Some countries also require that we provide your Tax ID with the customs documentation. To avoid delays we'd recommend that you provide us with this information once you've placed an order.
Our default terms of sale for shipments is CIP, 'Carriage Insurance Paid' meaning we are liable for the cost of shipping the item to you and for it's insurance, but not for any duties or taxes levvied against the shipment.