Each time you add a note to a ticket, you can choose to "notify staff" or "notify everyone". The note is embedded into the "Helpdesk ticket update email" email template. This is a great place to put extra customer service information such as expected delivery times, warranty and returns information, or even latest sales offers.
Edit the ticket email template at Settings > Templates > Document templates and choose the "Helpdesk ticket email update". The email subject will be the ticket title, so you don't need to change the one that's already there.
You can use the following widgets which will be replaced by the up to date information from within the ticket itself:
The full notes trail on the ticket. The email sent to staff will contain ALL notes, and the email sent to non-staff will only contain the notes that are marked as "public" (which is when you choose "notify everyone").
The ticket title
The original issue description
Who the ticket is currently assigned to
The current status of the ticket
The category, if selected
Who raised the ticket
The day the ticket was raised on
The time the ticket was raised