You can send an instant email reminder to your customer by search for and selecting your customer's invoice.
Click 'Prepare instant reminder':
The instant reminder uses the Reminder 1 default template and you have the option to amend the template for this specific email.
When you are happy with the template, click the Preview & Send tab:
You will see an option to send a preview to your email address or send directly to your customer. Click 'Send Reminder Now'.
Manual reminders and statements are recorded in the Sent box along with the automated emails.