I reckon adding “Customer Statement” in the InvoicePlane 2 would be a good feature and be able to email the pdf copy using email template like;
Dear %CustomerName%,
Its been a great experience working with you.
Attached with this email is a list of all transactions for the period between %StartDate% to %EndDate%.
If you have any questions, just drop us an email or call us and we will get back to you.