For VAT registrations in different countries, additional documents can be required. 

• In Spain, for the completion of the VIES registration, the Amazon seller agreement is needed. This is a document, by which Amazon confirms that you have accepted their Amazon Services Europe Business Solutions Agreement and that you have shown the intention to sell on additional Amazon EU websites. Please download the Amazon seller agreement below.
• This agreement has to be signed by Amazon and sent to your Account Manager. We will forward this to the Spanish Tax Authorities and your VIES registration in Spain can be completed.

To contact Amazon for this, please go to the Amazon Help Center, open a ticket for the support in the Tax and VAT section and send the sample for signature.