I wouldn't imagine either GOTY edition for either game will ever be on GameFly, at least the console versions that is. Both of those editions contain the codes for the DLC for each game, and to my knowledge GameFly doesn't support the sale of DLC for consoles at least at this point, only the retail game itself.
If you lose the slip for the game you need to report a shipping problem. Go to "My Account" and select "Report a Shipping/Purchase Problem", then select the game that you have lost the slip to and Gamefly will ship you a new one. If you have already sent the two games together in one slip, you'll want to inform a GameFly Customer Service person of this so they can assist in whatever way.
From what I've heard Dead Island will offer up to 4 player online co-op, and players will be able to join in and leave at will. As for split-screen co-op I have not heard any mention that it will be included in the game.