Welcome to the free In app purchases tutorial. I will update this thread as updates for the tweak come out.
This Is for educational purposes only :p

Okay noob friendly tutorial time!

Step 1: Open cydia.
Step 2: On the bottom ribbon tap manage.
Step 3: Tap sources.
Step 4: Up at the tap right tap Edit.
Step 5: Now at the top left Add should have appeared so tap that.
Step 6: Add this source system.in-appstore.com/repo/
Step 7: Once cydia finishes adding the source open up that source and the only package to install will be "LocalIAPStore" install that.
Step 8: Go over to settings and scroll down to localIAPstore and make sure both options are enabled.
Step 9: Now load up any app that has an in game store.
Step 10: for example lets do temple run 2.
Step 11: at the menu tap get more coins.
Step 12: Tap whichever amount you would like.
Step 13: It will add the amount to your account then immediately after a screen will pop up asking you to confirm your purchase HIT CANCEL NOT BUY.
step 13: You're done the purchase should be complete.

NOTES:

NOTE 1: This will work on pretty much any app that has an in app store That is not server based!

NOTE 2: Do not do this on an itunes account with a credit card set up on it. I am not responsible for any accidental purchases or anything that may happen along those lines.

NOTE 3: Make sure you go into your settings and enable the tweak as well as custom receipts.