Thanks to everyone who provided tips. I'm very appreciative. I do want to share a strange occurrence that happened to me in the app:
So, I play this app on an iPhone 4, which is unsupported. It plays all right, but it does lag and sometimes flat our crash. So, after grinding, grinding and spending a good $30 bucks on the game, I have finally obtained a Batman card via a booster pack, which would give me the final costume I wanted to earn. However, my app froze right before I could "confirm" my prize. So, I had to restart my app.
I turn it back on to reveal that my money was spent, and NO Batman card was in my inventory. In spite of that, I have still apparently unlocked the Batman Beyond costume, since my WBid confirms it.
So...I guess you don't need a Batman card in your inventory, you just need to "obtain" one? Hopefully it shows up when I link my WB account to the actual Injustice game.