Question: On my Xperia J phone Google Play Store setting of automatic updates is set to: Wi-fi only.

To my greatest surprise, yesterday it started to download 50MBs of updates when I was on 3G and my 200MB package was almost exceeded. 

I checked the Data usage of Google Play Store on my 3G network and on Wifi: Wifi has 432 MB traffic, **3G has 82 MB** (with wifi-only settings). 

My question is: what settings should I use to prevent the abuse of my 3G package? Have you experienced similar behavior?

Answer: 
If you haven't done so already, try going to `Settings - Data usage`, then press the menu key and choose "Restrict background data".

This sets all sync functions to only work on Wi-Fi, maybe it also helps with your Play Store problem. Be careful though, if you rely on sync to work even when you are not on Wi-Fi (Gmail for example), this is not the right option for you.
You can also make use of [No Root Firewall](https://play.google.com/store/apps/details?id=app.greyshirts.firewall) if you want only some apps to access the Internet from your phone if your device is not rooted. If your device is rooted you can use [AFWall+](https://play.google.com/store/apps/details?id=dev.ukanth.ufirewall)