I appreciate you for getting back to us, gaselec.

The option to modify the default body message when sending a remittance advice isn't available. What we can do for now is manually edit the message before sending the bill payment.

I'll take note of this idea and share it to our developers. They'll analyze this one and may be added to our future updates. 

If you have additional details, please let me know. I'll take if from there.