I would like to have the user be able to play my game from multiple devices without losing their inventory/in game cash. For this, it seems that I will need to use my own/a third party game server.

I have played games that have this functionality, yet they only require the player to login to their apple account (I.E., there is no second login for the game studio).

I am curious if anyone knows how you would handle this type of scenario. Essentially you would need to have some information that was consistent with the users account (and not their device) being returned to the server, yet I cannot find any method by which to use the apple id for my own purposes.

Is there perhaps something returned in apples store receipts? Has anyone else had any experience with this?

Thanks

It's quite simple I think. On your server you have to build MySQL database where UserID will be key for store items/gold/etc etc etc. When user start your game first time you have to connect to youe server and create new account with id. Next when user get new item use
network.request( ) ( http://developer.anscamobile.com/reference/index/networkrequest ) to send it to server and in PHP file use $_GET['blablabl'] or $_POST['aaaa'] to intercept game data.
Now you can use it in SQL like:

UPDATE Users SET gold = '1000' WHERE UserId = $_GET['1234'];

or get data into current game:

SELECT * FROM Users WHERE UserId = $_GET['1234']; and send it as JSON response to your game.