You don't need to buy again.
If you delete the app and re-install, or install on another device with a same Apple account, when you Share Videos(Share in iTunes/Save to camera/Extract audio) it will prompt and ask you to pay again, just click "Yes" and "Buy", it will show "You've already purchased this. Tap OK to download it again for free."
click "OK", it will unlock IAP item for free. You won't have to pay again.
Customer support service by UserEcho