How to request an invoice for Captions depends on how you purchased your Captions subscription.
If you paid using the payment method attached to your Apple ID (most people), please search your email for an email from Apple on the day that you signed up for Captions. Apple provides you with a receipt on the day that you signed up. If you can't find it or need to request another one:
Open Apple Settings app
Tap your name, then tap Media & Purchases
Tap View Account.
Tap Purchase History
Tap on the total billed > Tap Resend
Direct payment by credit or debit card
If you paid directly with a credit card at app.captions.ai, here's how to request an invoice:
Log in at app.captions.ai
Enter your email address
Click on the link sent to your email address
Scroll to the bottom and click on your most recent invoice