OnSong is an app you can download from the App Store. If you've purchased OnSong from the App Store, your payment is made directly with Apple. This means that the purchase history of OnSong and other apps you own are kept solely by Apple. Developers do not have the ability to access your personal or financial information. This means that if you have purchased OnSong through the App Store, you'll need to find that information through the App Store.
Accessing your App Store Purchases
To do this, open the App Store app on your device's Home Screen. Assuming you're signed into the Apple ID you used to purchase OnSong, tap on the account button in the upper right corner of the app to view your account details. In this screen you'll find a Purchases button. Tap on this and choose My Purchases to view the apps you've bought through the App Store. You can learn more by reading this Apple App Store support article.
Accessing Solo or Group Subscription Billing History
If you've subscribed to OnSong by creating a Solo or Group subscription through the OnSong website, you can access your billing history through the Billing Portal. To do this, sign into the Account page of the OnSong website and click on the Billing link on the left side. If you do not see a billing link, click on the Switch Account to choose the organization that you administer. The billing portal can only be accessed by users with administrative access to financial information.
Once you're in the billing portal, you'll see your billing history with the option to click to download invoices for each payment.