You will have to report this to GameFly and will be charged for the game. Split Second is a terrible game so the sucker who stole it didn't get away with much and can only resell it for maybe $8. And now you're stuck with the bill. You can try stating your case, but it's a losing battle.
My only suggestion would be to see what they are charging you for the game. Check online and with Gamestop to see if you can buy Split Second for cheaper, and send that one back to Game Fly. They won't know and it doesn't matter anyways.