In a nutshell:

You’re required to share Catawiki’s IOSS (Import Tax ID) number with the shipping company for applicable orders going to the EU. You can see if it applies and the IOSS number itself in the order details. Check with your shipping company the exact way you need to share the IOSS number. 

Professional sellers shipping from outside of the EU need to share our IOSS (Import Tax ID) number when shipping orders up to €150 to the EU. You can see both the orders this applies to and the IOSS number itself in the order details. For these orders, we use the IOSS (Import One-Stop Shop) to collect, declare, and pay the VAT to the relevant tax authorities. 

This is required to avoid shipping issues related to import rules. If you don’t share our IOSS number:

• A buyer can be charged VAT twice: by us at checkout and again by the shipping company when the order is delivered. 
• Some shipping companies won’t accept packages without an IOSS number for exports up to €150, which can delay shipping.

Both of these can lead to negative buyer feedback, having to compensate for additional costs, or restrictions on your account.

Check with your shipping company where exactly you need to share our IOSS number. It could be via a customs form, invoice, or somewhere else. The IOSS number might also be referred to as the Tax ID or VAT reg. number.