Go to your purchase page (not your account page, where you can get your keys/coupons/shows the total payments) and look for this:
You will see when you click it options to add money to your purchase, it will show how much you paid previously and then if you add enough to beat the next tier then the keys for the games from that tier will automatically add to your purchase page (keep in mind, the amount you add is ON TOP of what you previously paid, what you enter is not what you want to change your purchase amount to).