I received an email about a purchase I did not make.

Most likely there is no need to contact anyone as it is probable a phishing attempt. If the only indication of the charges is from an email or text, be aware that it may be a phishing attempt to get you to log in to a site that appears to be Apple and divulge your password or credit card information. See: Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

and Identify legitimate emails from the App Store or iTunes Store - Apple Support

You can use the following instructions to determine if it is a real charge, what it is for, and to report a problem: Get help with an item purchased from the App Store, iTunes Store, or iBooks Store - Apple Support

Also see: If you see 'itunes.com/bill' or an unfamiliar charge on your statement - Apple Support

It is likely a scam attempt. Do not respond to it nor provide them any personal or financial information.

You can check your purchase history here: See your purchase history in the App Store or iTunes Store - Apple Support

Check your iTunes Store purchase history, including hidden ones.

If it doesn’t appear, forward the email to reportphishing@apple.com as an attachment and then delete it.

If it does appear, change your password and ask for a refund.

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Odds are it is a scam...

read about these and decide for yourself

Phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support