If you are sending your shipment domestically(within the same country) or you are sending within the EU, your shipment will not need to go through any type of customs clearance and therefore do not require customs forms.
For all other shipments, you will require customs forms to accompany the shipment.
We provide you with a handy customs system in your My Orders. You may require additional documentation for your specific country. If we are aware of any specific forms per country, we will make you aware of this on your customs page.
Please be aware that customs forms are now required for all services to or from the UK (excluding domestic services and NI services).