Instead of going to Messages and composing a cancellation email within the booking, you can set your Collins system up to automatically send out a cancellation email when the status of the booking is changed to Rejected/Cancelled. Once this is set up, you can add a customised message to the automatic message.
You can add text to this message by going to Settings:
Please note the text box will only appear once automatic cancellation emails are ticked.
The text will then appear in your cancellation email like so: