If there's no option to send an invoice via email, you need to check the email settings have been configured.
To check these settings, please follow these steps:
Click Settings.
Hover over Other.
Click Email Settings.
Check the email settings have been set up correctly.
Click the Customer Invoice tab.
Ensure Customer Invoice box is ticked.
If this hasn't been previously configured and you need help to set up the email settings, this guide should provide assistance.