If you have a VAT number you can enter that when you purchase in the box that pops up when you select your country, and the VAT will be deducted from your purchase amount (As described on the payment form under VAT)
please don't include your land code (ES in the example below) in this section, it is filled in automatically.
You can validate your VAT number here: http://ec.europa.eu/taxation_customs/vies/.