According to EU legislation Graphical-Maps.com has to charge VAT. The VAT rate is based on which country the buyer is from. If the buyer is located in any of the following countries VAT has to be paid:

Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Poland, Portugal, Republic, Romania, Slovakia, Slovenia, Spain, Sweden And United Kingdom.

However if you have a business in any of the mentioned countries (except Denmark as hdri-locations.com is located in Denmark) and have a valid VAT Identification number you can be exempt from paying VAT using the Reverse charge rules.

When you supply the VAT ID it has to be in a certain format, it has to start with two letters followed by a series of numbers, the amount of numbers depends on which country you are from. You can see the format the VAT ID should have for each country here:
http://en.wikipedia.org/wiki/Value_added_tax_identification_number

If you believe you have a valid VAT ID but the site keeps saying it's invalid, then the there might be a problem with http://ec.europa.eu/taxation_customs/vies/ where the ID is getting validated, in that cases leave it blank and contact us at info@hdri-locations.com you can also check the page to see if it is in fact a valid ID or if there is a problem with it. The EU server is unfortunately not the most reliable in the world.