In order to avoid this kind of email:

"I've initially booked an afternoon slot for SERVICE PLAN SERVICE on 14th of May, but I can no longer make it. I've instead booked it for 21st May, afternoon. The portal doesn't allow me to cancel or reschedule it, and instead requires me to inform you manually (this seems like a poor design for a booking system), and hence this email."

We receive multiple emails of this nature per week so is a pain point for customers