I've been looking around for the best price at various sites and happened upon the lowest one yet on a game key site called 2Game, a legitimized key retailer in the UK that buys straight from the publisher much like GreenManGaming does.
ALL KEYS WORK IN THE USA
unless otherwise stated on the product page.
Add to cart and apply promo code: LOVE2GAME
to receive the discount.
As always with these type of foreign sites paying with paypal or your bank will convert to the necessary currency with ease. My code was delivered in 1 hour.