When supplying goods to Spanish consumers you need to decide if you are liable to register for VAT within Spain. For EU companies there is a distance selling threshold of €35,000 per annum, meaning you will not have to register for VAT in Spain until you reach this amount of sales to Spain from other EU countries. As for non-eu businesses, unfortunately the threshold is nil and you will need to register for VAT before you begin to make supplies to Spain. Other circumstances which trigger VAT registration would be if you store goods within Spain you would need to register for VAT before you begin to store goods and make your first sales
Apostille & Notary
As part of the registration process all documents including power of attorney are required to have the apostille stamp and signed by a notary in order to be accepted for registration, these are normally charged directly by the Notary and not by the VAT agent or tax office.
The positive of the Apostille stamp is that it legalises your documents and allows the tax office to register you for VAT in Spain much faster, generally once the documents are completed you can take them to the tax office in person and receive your VAT certificate immediately.