Please add an option to edit email notifications for invoices , overdue reminders, estimates, etc, using a rich text editor and HTML editor.
Also if you could create and save templates for these emails it would be great too. You could then assign templates to certain customers by default in a future update as an example. Zoho Books have a good implementation of this.
Add placeholders/shortcodes to the email section too. See example here with Xero: https://central.xero.com/s/article/Add-email-templates#Placeholdersyoucanuseforeachemailtype