It would be nice if Wild Apricot would automatically generate an invoice and send a notification for waitlist registrations which are completed manually. There is a button "Send notification" just below "Complete registration" (not shown in the help topic https://gethelp.wildapricot.com/en/articles/3-event-waitlists#gettingoffwaitlist), but this is gone once one has clicked "Complete registration". I didn't find an option to generate an invoice, and had to do this manually, as well as manually triggering the notification mail.