How do I send automatically generated invoices in the attachments of an e-mail?

You can add invoice templates as attachments to an e-mail template. To do this, go to the template of the relevant e-mail and click on Attachments (PDF). You’ll find this option in the grey panels on the right side of the screen.

You should be able to find the invoice template there. If not, the issue might be that the invoice template is set to the Invoice context. Since the e-mail is likely set in a different context (for example, reservation emails are typically in the Reservation context), the invoice template can't be accessed from the e-mail template.

To fix this, go to the invoice template and change the Context setting to General. Don’t forget to click Save at the bottom afterwards! If you now go back to the e-mail template and open Attachments (PDF) again, the invoice template should appear in the list.