Invoiced for something I never bought

I suspect that this is a scam.

Does it appear under your purchase history?: See your purchase history for the App Store or iTunes Store

Have you been charged?

Also see this, as it tells you how to identify whether the email is legitimate and tells you how to report the scam: Identify legitimate emails from the App Store or iTunes Store

I'd also suggest checking with Smule regarding this, just to make sure this wasn't sent by them.

Apple will always address you by your name or the name they have on file for you, not Dear Customer, Dear Client or by using your e-mail address. The e-mail will be from @apple.com or @iTunes.com. E-mail addresses can be spoofed. You can go to Mail/View/Message/Show all Headers to see more. Apple e-mails will never contain an attachment. Apple will never request personal information by email such as Social Security numbers, your Mother’s maiden name or full credit card numbers .

The only exception to the above I have noticed is if you order something from the Apple Store (apple.com), your receipt will be addressed to Dear Apple Customer. That is a receipt for a purchase you initiated.\

Avoid phishing emails, fake ‘virus‘ alerts, phony support calls, and other scams. https://support.apple.com/en-us/HT204759

Send the e-mail to Apple as an attachment to a new e-mail before deleting it. You can forward as an attachment by going to Mail/Message/Forward as attachment. Or control - click on the email and select Forward as attachment. Make sure you send it as an attachment to a new email. If you just forward it, it will probably be rejected. You won’t receive a response.

reportphishing@apple.com

That is a scam. It doesn't address you by name and it contains a PDF attachment. Apple emails address you by your name and don't contain attachments.