A customer says they did not receive the invoice

The invoice email goes to the billing email saved on the customer record and nowhere else, so check that address first, then ask them to look in spam. You can also share the document as a link instead of an email — the share page shows it and, where online payments are set up, lets them pay from it without needing an account. The link carries an expiry you set when you generate it, and you can rotate it to invalidate the old one.