2dguy What's the easiest way to store/retrieve the user's current number of coins?
I wonder if I could use Google Play Services to store the user's coins using their "saved games" feature? The only thing the game would save would be a number representing the # of coins.
I believe this was the original question. I am sorry I dont see any reference to IAP. Even if there was a reference to IAP, my recommendation stands for itself.
2dguy me wonder if they also rolled their own currency purchasing, etc, using Firebase.
Seriously, I have no idea what you talking about? I believe I have not made any comments about Firebase that can draw such conclusions.
Let me try once agin,
If you have a player with player_id = Player01, who has 100 Coins in his inventory.
All you have to do is to create a database in Firebase called MyGameDatabase.
Inside the database create a node called Players and add the data using his player_id, like this : -

After that, every time the player loads the app, just retrieve the data for the player_id - Player01 in order to get latest data. When the player plays one round, update the node values accordingly.

When the player makes an IAP or earns coins from the game by winning or by watching ads, you can update the node value by incrementing the coins value accordingly.

If you want to retain the player data even after an uninstalls & reinstalls, you can use Firebase's login system which is automatically liked with the database. In that case the player_id would be automatically generated by Firebase.
If you want to keep it anonymous, then you need to get the device ID and use it instead of Player01. The Device ID would remain same even after the app is uninstalled and later reinstalled.
I hope, this time I was able to convey the message.
If you honestly ask me, I would recommend GPS only if you are targeting Android alone. Otherwise, for me it is a bad choice.