Yeah, that is how I thought it worked. But in my experience cards disappeared from the upgrade screen with no rhyme or reason, and upgrades seemed to only be possible when you had 3 or more of the same card, even though you only need 2 to combine.
Oh, is that what it meant? I interpreted it as buy a key to skip, not merely retry. Since that would mean making the game harder for bad players, it didn't even occur to me.