Requirements:
1. A Jailbroken Device
2. A brain
If you haven't jailbroken your iDevice yet, install it with Cydia Impactor and the file from https://yalu.qwertyoruiop.com/
This works for iOS 10 and older jailbroken devices as well!
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Instructions:
1. Open up Cydia and add the source (http://repo.hackyouriphone.org/)
2. Wait for it to install and then search up LocalIAPStore and install that as well.
3. After installing, open up settings and scroll down until you see LocalIAPStore and then press on enable.
4. Pick on an app then press the area where you are suppose to purchase packages or items, then wait for it to load until Apple asks for your password. Just press cancel and it should show up that you 'purchased' it.
WARNING: This may not work with some other apps, but majority should work just fine!
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If you have any questions, please dont be afraid to ask!