Infernalist: System is stupid as fuck. I entered the code, selected all five games I wanted discounted, it applied the discount and then immediately lost the discount when I went to buy them.
Stupid system, couldn't reapply the discount, so fuck it. Lost a sale.
That's how it works.....if you hit confirm/continue twice(after being prompted each time) on a code it applies the code to your cart and you must take advantage of it then(as the code in "used up").
If you want to save a code and use it a bit later(while it is valid), make sure to not confirm the code more than once(to just check what the discount is, but not apply it to your cart items) until you have all the items you want in your cart, money on your payment source of choice, and are ready to purchase/check out.
