If you are using OnSong on a different device than you originally purchased in-app purchases, it may appear that you must purchase them again. This happens because apps are not permitted to know what you have purchased without going through the purchase process, or restoring past purchases.
If you were to tap to buy again, you will be prompted to sign into your Apple ID. At that point you would be shown that you've already purchased the add-on and can reenable it for free.
Alternately, you can go into the Utilities Menu by tapping on the gear icon in the Menubar and then choose Account. Scroll to the bottom of the screen and tap Sign Out if you are signed into account. This is because once you move to using an account, your features are determined by your membership tier. Signing out does not effect your OnSong library or settings.
Now there should appear an Add-Ons row at the top of the screen. Tap on this to view available add-ons. Tap on the Restore Past Purchases button at the bottom of the screen. This will request the App Store to retrieve purchases you've made through OnSong and then reenable all your purchased add-ons.
Now there should appear an Add-Ons row at the top of the screen. Tap on this to view available add-ons. Tap on the Restore Past Purchases button at the bottom of the screen. This will request the App Store to retrieve purchases you've made through OnSong and then reenable all your purchased add-ons.
It is important to note that if you are using a different Apple ID now than the Apple ID that you used to originally purchase the add-ons, then you will be prompted to buy the add-ons again for your current Apple ID. You might be able to merge purchases like these between Apple ID's if this is the case, but this is something that OnSong has no control over. You would need to contact Apple directly to inquire about this.
If you have issues with any purchase-related issues, please contact Apple App Store support as all purchases are made directly with Apple and developers do not have access to your account history. You can contact Apple App Store support at http://onsongapp.com/support/appstore