According to EU legislation cg-source.com has to charge VAT if the buyer is located in any of the following countries:
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.
The VAT rate is from Jan. 1st. 2015 set to the VAT rate of the country the buyer is located in.
However if you have a business in any of the mentioned countries (except Denmark as cg-source.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 you supply the VAT ID in the form 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 firstname.lastname@example.org you can also check the page to see if it is in fact a valid ID or if there is a problem with it.