Send on an invoice now opens a panel beside it with the email it will send. To starts with the contact's primary address, and you can remove it, add more, or add Bcc addresses the others will not see. The subject and message start with your team's wording, with the customer's name, the amount due and the due date filled in, and you can change them before sending. Everyone in To gets one email.
Send test to me sends you the same email with "[Test]" before the subject. Change the message and send another test as often as you need; a draft stays a draft.
Payment reminders email the customer about what is still to be paid, on the due date and then 7 and 14 days after, until the invoice is paid in full or voided. Send starts with them ticked, and each invoice shows when the next one goes out, with a switch to turn them off. Change the days, the reminder message and the invoice message in Invoicing settings.
Open in PDF Editor, Duplicate, Void invoice and the other actions on an invoice are now in its More menu.