http://www.gog.com/en/page/tw2v3

Since the EU price is almost $16 more than the price in USD, we're giving our EU customers a $16 credit to purchase anything on GOG.com.

What is GOG.com's Fair Price Package?

Gamers who are paying in euros are generally charged more for the games that they buy than gamers in the US. Even if they're charged the same number in euros, since the exchange rate favors them over the US dollar gamers in those regions are still paying more. That's not fair, and GOG.com believes that charging the same price worldwide for all customers makes more sense. Unfortunately, we can't just lower our prices on the preorder to be lower than everyone else in those countries, due to legal reasons. So instead, we've come up with a solution that lets us charge a fair price to everyone while avoiding giving the legal team here at GOG.com a heart attack.

I'm an EU citizen. Why do I have to pay more for The Witcher 2?

Due to licensing and legal reasons the price of The Witcher 2 Digital Premium Edition in the European Union was set at 49.99 euros. The preorder price, being 10% less than that, is 44.99. Due to the favorable euro exchange, that means that EU customers are paying roughly $16 USD more than customers in the rest of the world, which just didn't seem fair to us. We're always trying to get the best value for money to our users, so to compensate our EU customers for the price difference between European Union and the rest of the world, we'll be giving all EU customers a $16 USD credit to spend on anything at GOG.com.