Invoice availability depends on where you purchased your subscription.
Web purchases
If you subscribed through https://start.getfitx.app, invoices are sent to your registered email address after each successful payment.
You can also review your subscription details by signing in to your account at:
Apple App Store
Apple manages all payments and invoices for App Store subscriptions.
You can access your receipts through your Apple account.
Google Play Store
Google manages all payments and invoices for Google Play subscriptions.
Your purchase history and receipts are available through your Google account.