Hi Tiina,
Thanks for your suggestion. I understand the problem that you are facing. As you suggest, the best way to do this at the moment would be to take the invoices you wish to chase and put them into your own email to send.
The automatic reminders will not facilitate this and this isn't something that we are currently looking at altering unfortunately.
KashFlow Product Team
The biggest benefit of sending multiple reminders is that you can collect interest and compensation on each of the overdue invoices.
Sending reminders is not rude. Not paying for goods or services is in fact rude.
When a customer has more than one overdue invoice then you should be able to send just one e-mail attaching all outstanding rather than having to send multiple e-mails to the same customer with just one attachment.