here is an email template for sales invoice i am using which other users may find useful.
Subject
Invoice Number {{reference}}
Message body
Greetings M/s. {{ recipient.name }},<br><br>Please find herewith attached your Invoice Number {{reference}} dated {% for field in fields %}{% if field.label == 'Invoice date' %}{{ field.text }}{% endif %}{% endfor %}<br>The Invoice value is {% for total in table.totals %}{% if total.label == 'Total' %}<b>{{ total.text }}</b>{% endif %}{% endfor %}<br>{% for total in table.totals %}{% if total.label == 'Balance due' %}{% if total.text !='' %}The balance amount due is <b>{{ total.text }}</b>{% endif %}{% endif %}{% endfor %}<br><br>If payment not made already, please make payment to the below bank account.<br><br>**Your bank details here**
this template gets the following details necessary for a customer:
Invoice number
Invoice date
Invoice value
Balance due
Bank details