When I want to buy it at GreenManGaming I should use the 25% voucher, right? Or is there a better promo?
You can't - it's a $50 game, so they discounted it by 10%/$5, which means you can't apply the coupon (which makes a $60 pc game into... yep, $45 also)
Long story short, it's $45, unless you can find another deal somewhere else.